BT 2+8

Introduced in July 1991, this system replaced Ambassador, Escort and Minimaster 3 from BT's portfolio of telephone systems.

APPROVAL No S/147013/L/502065

User Guide

Programming Guide

The BT 2+8 is a microprocessor controlled self contained switching system which can be configured on site as a PABX or Keysystem, there are two versions of CCU, Original BT 2+8 and 'Enhanced BT 2+8'. Both systems support up to 2 exchange lines with up to 8 extensions and offer normal modern PBX/Keysystem facilities. The system is 'through fed' on Exchange Line calls hence extension terminals must be configured for MF or LD signalling according to the type of signalling supported by the exchange line / host system.

The 'Enhanced BT 2+8' is externally similar in appearance to the 'Original' CCU but has a redesigned circuit board which offers various additional facilities.

Extension telephones may be any BS 6317 approved telephone with earth recall facility and/or 2+8 Systemphones (in any combination but at least one Systemphone is recommended on original BT 2+8 and required for approval purposes on Enhanced BT 2+8). Systemphones may be used as an Operators Terminal in PBX mode or Key Terminal in Key system mode. Later production Systemphones have improved Handsfree performance and will incorporate a stored number memory protection battery.

A 2+8 systemphone is shown on the right.

2 Exchange lines and 8 extension switch.
2 Intercom paths for internal calls. (LD & MF to Exchange)
Power fail Extn 20 to line 1, Extn 21 to line 2.
Power consumption - 20 watts.

Facilities include:-
- Press recall button or dial 41/42 access to exchange lines.
- LED indication of line/extension status
- Last number redial
- Microphone mute/secrecy button
- On-hook dialling with Handsfree LST operation
- Ringer volume control
- Abbreviated dialling
- Speak (Voice announcement to other systemphones)
- Dual signalling (MF/LD switch on base + soft-switch to MF by pressing
- Divert
- Ringer on/off

Additional/changes to facilities offered by `Enhanced BT 2+8':-
- Exchange line access by dialling digit 9
- Four levels of call barring with customer programmable exceptions
- Music on hold interface
- Extension self test
- Exclusive line changes (for Fax machine use), line 2 will only ring extn 21 but all extensions have access to both lines (I/C & 0/G)
- Recall to host PBX by pressing the recall button and then dialling 'digit 71
- Warn Tone no longer applied on external calls
- Off Premises extension (approval pending for external extensions via the Network)

Many facilities involve the use of a Systemphone

Made by Philips BCS Limited

Approved for direct connection to the PSTN (loop disconnect or MF4 signalling) or by working as a subsidiary to a host PBX.

BT 2+8 CCU is intended for installation and use in a normal home or office environment on a dry flat wall, it measures not greater than 550 x 320 x 80mm and weighs no greater than weight 5Kg and should be mounted at a convenient working height. The normal limits are usually considered to be, minimum height, bottom of CCU to the floor 1 metre, and the maximum height to the top of the CCU should not exceed 1.75 metres. The CCU is intended to be mounted with the cover fixing screws at the bottom edge, it should not be mounted upright.

Cooling of the CCU is by natural convection, therefore it should not be installed in a confined space and it must have a minimum clearance of 100 mm on all sides with sufficient space to access cover fixing screws.

The CCU should not be sited where it may be subjected to excessive levels of heat, dust, damp or high humidity. Locations near sources of electromagnetic radiation such as heavy electrical switch gear (e.g. lift machinery or electric arc welders) should be specifically avoided.

A Mains plug and 240 Volt, 13 Amp, 50 Hz mains power socket within 2.5 metres of CCU dedicated to the system will be required. A mains conditioning unit (MCU) should not be necessary as the CCU incorporates a mains filter.

A 1.5mm sq. functional earth will need to be provided if:-
(a) The system is subsidiary connected to a host system that requires Earth Loop Recall.

(b) There is a risk of high voltage line surges.
If a functional earth is provided for (a) or (b) above it may be used for Electro Static Precautions.

There is no provision for standby power, Lines 1 & 2 will automatically switch through to extensions 20 & 21, respectively under power fail conditions.

CCU to extension cabling limits (using cable 2503) are:-
25 ohms (approx. 150 metres cabling) for a Systemphone on Original or Enhanced BT 2+8 CCU.
50 ohms (approx. 300 metres cabling) for a standard BS 6317 telephone on Original or Enhanced BT 2+8 CCU (3 wire extension).
336 ohms (approx. 2Km cabling) for a standard BS 6317 telephone on Enhanced BT 2+8 CCU (2 wire extension).

Each (internal) extension (regardless of Systemphone or type of telephone used) must be (6-wire) cabled between the extension terminal strips in the CCU and Master Line Jack at extensions, all 6 conductors must be terminated at Master Line Jacks as per standard wiring practice.

Although earth recall telephones are specified, the earth recall lead (LJU 4) must be terminated on the extension terminal strip in the CCU and not to earth, only the spare (orange/White) conductor is not used and must be insulated and left spare in the CCU.
Multipair cables (equivalent to cable 2503) can be used for extension cabling provided each extension is connected up by with 3 (twisted) pairs. Pairs must not be split.

The extension ports have an output REN of 4 and Extension Bells / Tone callers can be hardwired connected to LJU terminals 3 and 5, and additional telephones via parallel connected secondary LJU's (but only one Systemphone per extension).

The Enhanced BT 2+8 CCU contains a Lithium battery which must not be short circuited as this may cause the battery to burst or leak, also a heat sink which can remain uncomfortably hot for a few minutes after power is switched off.

The mains lead is part of the BT 2+8 Power Supply Unit (PSU) and must not be shortened (unless necessary for re-terminating the mains plug). Any excess mains lead should be neatly coiled away, if the mains lead is shortened, it may invalidate the manufacturers system warranty.

The `Enhanced BT 2+8' is supplied with metal cover fixing screws. These are intended for permanent use and are not to be replaced by the plastic screws used on original BT 2+8 covers.

Extensions 20 & 21 will not initialise and `Line busy' LED(S) will be lit on Systemphones unless the exchange line(s) are connected (or on single line systems, the appropriate SW1 switch is operated).

LJU wiring

CCU Colour LJU 2/1A
1 Green/White 1
2 Blue/White 2
not fitted Orange/White 3
4 White/Orange 4
5 White/Blue 5
6 White/Green 6

Only 5 wires are connected at the CCU end

Power Fail wiring

Line Extension
1 20
2 21

Centre and either outer IDC.

To prevent crosstalk and interference the pairs should not be split and the spare wire of the cable pair should not be used.

The REN of each extension port is 4, but only one Systemphone per extension.

The intercom current feed is in the range 22mA - 28 mA.

External extensions should use extension 27 in preference as this port has extra protection against high voltage surges. Any extension port can be used for an external extension if suitable protection is provided. It is also permissible for signalling converters to be used.

You will lose the hold and recall facilities on 2-wire external extensions. This will mean that once a call has been put through there will be no way to hold or transfer the call.

For safety reasons the CCU must be disconnected from the power supply whenever the CCU cover is removed. Altering the facility switches will have no effect until power is switched off and on again as the switch settings are only read by the system microprocessor when powering up. Any customer programmed facilities, including memory numbers stored in systemphones will be reset when the power is disconnected.

The Enhanced BT 2+8 contains an on board Lithium battery to preserve customer programmed call barring details, diverts, night service or bell-off programming. This battery must be switched on during installation and has approx. 10 year in service life (provided system is kept powered up).

New units for initial supply or for use as maintenance spares carry a 24 month warranty from the date of manufacture as shown by the first 4 digits (YY WW) of the 11 digit serial number marked on the back of the unit.

The Lithium Battery in Enhanced BT 2+8 CCU should be switched off on any CCU being recovered, stored or disconnected from mains power for any length of time.

Description                                                 BT Item code
BT 2+8 CCU complete                                         870446
BT 2+8 Systemphone                                          870447
Enhanced BT 2+8 CCU complete                                870512
BT 2+8 Systemphone (with improved Handsfree)                870513
Wall Bracket (for Systemphones) Clear                       870548
Fuse 72/1 (Antisurge)                                       313873
Fuse 88A/3 (Mains Plug Top)                                 314555


Capacitor-loop test conditions apply (value of capacitance may test less than a normal DEL terminated by NTES).

(a) Extension telephones - Not all modern telephones with an earth recall button will work on Original or Enhanced BT 2+8.

(b) BT 2+8 monitors the exchange line voltage to determine if the line is free, the line voltage links should be set to '50 Volts or 24 Volts depending on the normal `line idle' voltage. `Line Busy' problems may arise on systems where the line voltage can fluctuate between both of these values e.g. Gfeller Local Line Concentrators (where line free condition may be 20 or 50 Volts) or with WB 1400 systems where the line charged battery can reduce line voltage on long lines, these may be resolved by setting the links in the 24 Volt position.
N.B. the links have been repositioned on Enhanced BT 2+8 but are still supplied in the 50 Volt setting.

(c) Systemphones should not be plugged into the back of add-on units (e.g. Wren or Kingfisher II) where Line Cord conductors 1&6 are interrupted, a 6 way plug-in doubler may be used instead to connect the unit.

(d) Incoming line polarity is important when Original BT 2+8 is connected to the extensions of an Earth recall PBX, the incoming line (A & B wires) may need to be reversed to prevent call drop out when trying to recall the host PBX with certain types of extension telephone (this problem has been overcome on Enhanced BT 2+8).

(e) In the event of mains power failure/disconnection any programmed facilities will be lost on original BT 2+8 CCUs .

(f) Systemphones require approx. 15 - 60 seconds to power-up and handset must be lifted and replaced to initialise the handsfree facility before first use, stored numbers will be lost on mains power failure (later models will incorporate a memory protection battery).

(g) Systemphones may automatically switch to handsfree due to RFI (overcome on Enhanced BT 2+8).

(h) The bells of any telephones (or extension bells) connected to the same extension as a Systemphone may tinkle whilst Systemphone dials out (overcome on later systemphones).

(i) The extension self test facility on Enhanced BT 2+8 is intended to be used as a confidence test to avoid unnecessary change outs where a fault complaint cannot be reproduced, as it involves changing the Pulse/Tone signalling switch position on Systemphones, the test should be used with caution.

Some confusion has arisen following the recent modification (around mid 1994) of 2+8E units identified by ‘serial numbers greater than 0005019 and manufacturer’s dates of issue on or after year 93, week 21’.

As the manufacturer repeats the serial number sequence from January of each year, modified units with serial numbers lower than 0005019 are now available, but these can be identified by a date of issue of 94/01 or later.

Modified units can therefore be identified by:
date of issue: 93/2 1 (or later) Serial Number: > 0005019, or date of issue: 94/0 1 (or later) Serial Number: > Any

All CCU’s, whether new or refurbished, are issued with a new serial number and issue date before factory despatch.


There could be a problem with wall mounted Systemphones not ringing to internal or external calls. Investigation shows that if the handset is replaced lightly, the microswitch does not make contact, whereas if the handset is replaced firmly, contact is made and the instrument rings.

There have been reports of Mk2 Systemphones going into handsfree during conversation and the investigation proved the fault to the instruments. It was noted that the serial numbers of both Systemphones commenced with 9426... and it is considered that there could be a batch fault.

The suspected cause of the problem is RH, and this indicates that a batch of Mk2 Systemphones have poor RH immunity. As the fault only seems to occur when the Systemphone is on an exchange line, it is suggested to fit a BT80A RF2 to the exchange lines first, before changing the Systemphone.

Problems reported as detailed below are now fixed in V1.7 software.

• Extensions barred outgoing - they receive NU after 4th or 5th digit.
• Programmed ringing lost - bell off on line 1 for xtn 20 & line 2 for xtn 21.
• Exchange lines testing tele loop and/or locked up.
• Extensions buzzing or high pitched tone for dial tone.

These problems are caused by short breaks, between 30-300ms, in the mains supply and can usually be cleared by defaulting the system.

The recommended fix for sites experiencing any of the listed problems is to replace the software chip, thereby upgrading to V1.7 software.

Any sites that still suffer problems with the new software installed will probably require an additional hardware modification. This can be achieved by changing the complete CCU. Modified CCUs will be identifiable by serial numbers greater than 94390013582 on the silver label inside the CCU and by box labels marked ‘V 1.7 Software’.

The products listed below are not compatible with the BT2+8:-
Response 20, 50 & 150
Relate100 & 200
Vanguard Basic & 10E

Some faults may require the power to be turned off for at least one minute before the CCU completely resets. This is also the case if the battery switch (SW7) is placed in the down position whilst the power is off to clear all system memory. Bear in mind that if either of the two exchange lines are left disconnected when the power is restored, extensions 20 & 21 will lock-up with no internal dial tone/PET. The extensions will be re-instated when the exchange lines are reconnected.

There have been numerous enquiries regarding the connection of fax machines to the extension ports of a BT 2+8. Operating difficulties can be encountered as follows :-

Extensions have to answer an incoming call and identify it as a fax call before attempting to transfer it to the fax extension. If there is too long a delay in this being done, the sending fax machine may time-out, as it has not managed to “handshake” with the receiving fax machine within the required period after CSA (Called Sub. Answer). This results in complaints of unable to receive incoming faxes.

On the original 2+8 (and Renown), the extension must first seize a line by pressing Recall. Most fax machines do not support Earth Recall and are also not able to use the recall facility of any associated telephone plugged into its PSTN port. The 2+8E allows extensions to access lines by dialling ‘9’ and therefore overcomes this problem, but any Combined Tele/Fax machine must also support Earth Recall otherwise system facilities (i.e. Hold and Transfer) are lost when using it in telephone mode.

Both versions of the 2+8 and the Renown generate their own ringing to the extension ports. This ringing is made up of a square wave signal and is undetectable by some fax machines, i.e. the fax will not recognise the ringing current as ringing and therefore not auto-answer after the programmed number of rings. Problems can also arise with the difference in ringing cadence unless the fax machine has the facility to work on a ‘Private Network’. This allows it to more easily count local generated ringing.

A report of overhearing on a site with two external extensions has been received, and tests on a captive system have confirmed that under certain conditions, very low levels of speech can cross onto the intercom paths.

Both versions of CCU can be affected and the fault is more noticeable if both external extensions and internal extensions are on intercom calls. It is at a much reduced level on exchange line calls but increased considerably if there is any earth leakage on the external pair.

This fault should not be confused with overhearing ringing whilst on an internal call as detailed in Tech News 7 as this is regarded as a system facility.

The level of overhearing is very low and appears to some extent to be altered by the impedance characteristics of the circuit. If this problem is encountered, normal DC tests should be undertaken to ensure there is no earth leakage or cross coupling from unwound or mismatched pairs etc.

There have been reports of difficulty making outgoing calls from payphones on BT2+8 extensions. Both Mkl and Enhanced versions of CCU are affected.

This problem is caused by the BT2+8 internal dial tone timing out before the first digit is sent from the payphone.

In order to maximise availability of the 2 internal intercom paths, BT2+8 times out dial tone after 8 seconds if nothing is dialled.

When the payphone handset is lifted, it immediately seizes dial tone but takes a further 2 seconds to power up and request that money is inserted. The user now has to insert some money before dialling can commence, but the payphone will not transmit these digits, (with a leading ‘9 [pause]’ to seize the line), until it has validated the number dialled. This can take up to 4 digits depending on the route.

It is not surprising that a number of customers have found it difficult to do the above before the BT2+8 times out.

Due to the possible difficulties it is recommended that payphones are not used on BT2+8.

There has been an instance recently of a BT2+8 exchange line circuit being piggybacked off of a Senator external extension adaptor and the circuit failed to work. This is due to the fact that the two systems are not compatible working as a host/subsidiary. Although OFTEL have made observance of the host/subsidiary connection approvals advisory only, people should ignore them at their peril. Ambassador and Senator systems are not approved as host switches because their 2 wire circuits do not meet, and were not asked by BT to meet, the required standard.

The recommendation is that these switches should not be offered as host switches but if the customer insists, then they must be informed that the connection is made at their risk.

It is likely that the Ambassador/Senator external extension adaptor is incompatible with a range of systems - not just the BT2+8.

Some complaints have been received of ‘false busy when calling on-hook extensions’. The following scenarios can arise from the design of the switch which, it must be stressed, are not fault situations.

1. While an extension is audibly ringing to an external call, the system sees it as engaged and any attempted internal call to that extn will receive busy tone.

Particular problems can arise in Keysystem mode (or Night Service) if two external calls arrive but only one is answered.

In this case the answered call cannot be transferred to any extension still audibly ringing to the unanswered external call. (Although it can be transferred to any extension programmed as ‘ringer off).

Normally, the second call will be answered elsewhere and thus free the remaining ‘audibly ringing extensions’.

2. A similar situation can arise in operator mode if an extension attempts to call the operator while an incoming call remains unanswered. Until the call is answered and processed (or is abandoned), calling extns will receive busy tone. In practical terms this should cause no difficulty as external calls should only remain unanswered for short periods if the operator is free and in attendance.

Both 2+8 and 2+8E systems are susceptible to this perceived problem.

Enhanced versions are now available of the Converse 200, Relate 200 and Vanguard 10E with pre-programmed network service keys for Call Waiting, Call Diversion, Charge Advice and Reminder Call. Any of the keys offering a non-required facility can be overwritten with an alternative network service code or used as a memory store for any PSTN number.

Use of each of these items is. however, restricted if connected to extension ports of Renown, 2+8 or 2+8E systems. These limitations are as follows:

Converse 200 or Vanguard 10E Enhanced - network services are not accessible by these keys. Only reprogramming to PSTN numbers will be effective. Any required network services must be accessed from the keypad.

Relate 200 Enhanced - Reprogramming of Charge Advice and Call Waiting is necessary. To set for Charge Advice, enter RECALL, RECALL, PAUSE, *‘, 40, #. To set for Call Waiting, enter RECALL, RECALL, PAUSE, 2.

From week 3 3/93 all Converse 200 & 300 telephones should be fully compatible with Renown and 2+8 systems. From that date all instruments will incorporate the following modifications:-
1) Mechanical Recall switch.
2) Tone caller which operates to lower input voltage.

A side effect of ‘2’ is that the overall audible ringing level has been slightly reduced. This may cause comparison problems where teles are changed and the new model replaces the old. The RENs are unchanged.

Some feedback has indicated that the operation of the new recall key on the Converse 300 may be unreliable. The modified version can be identified by its week of manufacture: 33/93 or later.

Various complaints have arisen from 2+8 sites subject to radio frequency interference. These problems include intermittent flashing of LED’s, Systemphones switching to ‘Handsfree’ and overhearing local mobile radio.

If the problem is apparent only on exchange line calls, the problem can usually be overcome by fitting BT8OA RF2 filters or, if appropriate, replacing a Mk l Systemphone with the Mk 2 version. In extreme cases both actions may be necessary.

It must be stressed that RH problems affect only the 2+8 Systemphones particularly the Mk l version - and NOT the 2+8 CCU. There are no known RH problems with the 2+8 CCU and it should never be upgraded to a 2+8E to cure problems caused by suspected radio interference.

All refurbished Systemphones (item code 944935) should be of the Mk 2 version.

To verify an exchanged Systemphone, check the first four digits of the serial number. On Mk 2 Systemphones these digits exceed 9240.

A possible problem has recently been highlighted which initially appears as a loop line fault and is subsequently cleared by a power reset of the system. Both Mk l and Enhanced 2+8 systems may be affected, but evidence that a general fault exists is still inconclusive.

To establish the extent of the problem, Technical Support would like details of sites and specific fault conditions where an apparent line fault is cleared by a system reset.

New and refurbished modified systems are now reaching stock points. These can be identified by serial numbers greater than 0005019 and manufacturer’s dates of issue on or after year 93, week 21 (i.e. 93/21 or greater). Serial numbers and issue dates can be found on silver labels on both the external packaging and within the CCU.

As the maintenance item code is identical for both modified and unmodified 2+8E CCU’s, please ensure the correct stores are to hand before attending site.

Reports are coming in of Mk l units being delivered by courier in lieu of Enhanced versions. This further complicates the situation outlined in the previous article as the external appearance of the original 2+8 system is identical to the Enhanced and its serial numbering uses the same number range.

As a 2+8E CCU should never be maintenance exchanged for a 2+8 Mk l unit (or vice-versa), a visual check of the internal components may be necessary to establish the correct replacement item has been provided.
The maintenance item codes for each item are:
2+8 Mk l - 944934
2+8E - 946013

It has become known that some 2+8E extensions become barred to outgoing calls. The problem manifests itself as ‘receiving NU after the 4th or 5th digit even though no barring has been applied’. Additionally, it now appears that the barred extensions are often removed from the ring maps of both lines and diverts may come on by themselves.

This problem has now been traced to a faulty batch produced during Christmas week, 1992. The instruments can be identified by the first four digits of the serial number - in this case those beginning 9252. There is no evidence of other batches being affected 

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Last revised: November 23, 2021