|Systems 1 and 3 described in
the following pages represent simple exchange-connected extension plans, providing a
dependable telephone service.
They are particularly suitable for small organisations, and fulfil all main requirements for extension
plans covering secretarial, parallel extension, external-extension and dual
exchange-line working. Connection can be made to any type of automatic or C.B. exchange,
public or private.
The Plan Etelphone
The Plan Etelphone is the basic instrument for all extension plans, and is a variant of the Etelphone
(see catalogue sheet No. 7145).
Desk and wall-mounted versions are available.
The instrument case and the handset are moulded in high-impact plastic and produced in black,
ivory and two-tone grey. Colours are virtually light-fast, ensuring a close colour match of replacement
The handset cord is the coiled extensible type, sheathed in p.v.c. and coloured to match the telephone. Instrument colour
matching also applies to the desk block and its associated line cord, provided the number of line-cord
leads does not exceed six. In
excess of this number the desk block and line cord are provided in elephant-grey.
A carrying handle and performance
regulator unit are included in the
The dial (when fitted) has a clear plastic finger plate, standard numbering (i.e. 1 to 0)
and transmits 10 p/s with 2:1 break/make pulse ratio. Alternatively, dials can be
supplied with any desired numbering, pulse speed and ratio.
Press buttons arranged in line above the dial are locking and/or non-locking and up to four in number.
Locking pressbuttons are arranged
Automatic release after depression of the switch-hook or another in-line locking press-button.
Manual release by means of an
integral sliding cap. (Note: a pressbutton with this action provides simple on/off switching for
such purposes as instrument bell control.)
A fifth pressbutton is incorporated
to the right above the dial when transfer or recall facilities are required.
In-line pressbuttons are forest green on ivory telephones and
elephant-grey on black and two tone grey instruments. Dummy
pressbuttons, when fitted, are the
same colour as the handset.
Visual signalling is provided, the lamp lens being accommodated in the left-hand area of the case
above the dial.
Audible signalling is by a.c. bell
and buzzer. Either or both units may be incorporated in the telephone according to the extension-plan requirements and can be
supplemented by similar wall mounted units.
The a.c. bell signals incoming calls
from the main exchange. In-built transistor ringing generators provide a.c. ringing current for external-extension
d.c. buzzer signals calls between internal extensions situated in or near the same building.
Line limits between internal extensions with 6V local signalling allow
for loop resistances of up to 1000 ohms, this being equivalent, for example,
to approximately 200ft (61m) of 6.51b/mile (0.5mm) conductor.
Between an external extension
and the main exchange the loop resistance may be up to 1000 ohms.
Tropical finish with coils suitably impregnated and metal parts specially finished.
Handset amplifier for users with impaired hearing. Gives finger-tip
control of the level of reception (see catalogue sheet No. 7150).
Operator recall enabling the user
engaged on an exchange call to
recall the PBX operator for enquiry or call-transfer purposes.
Conversion parts to equip c.b.
instruments for auto working and
table instruments for wall mounting.
When required, local power supplies for signalling and transmission can be derived from
dry battery, or a 6V, 1A power unit connected to the mains supply
(100 to 110V or 200 to 250V a.c.).
AN INTERMEDIATE TELEPHONE
WITH EXTENSION SWITCHING
AND INTERNAL EXTENSIONS
HAVING DIRECT EXCHANGE
Plan system is suitable for up to two executives served by one exchange line and
requiring incoming calls to be filtered at an intermediate (secretarial) point.
Basically similar to BPO plans 105 and 105A, it provides two significant service
features: (a) the automatic holding of exchange calls at the secretary's
intermediate telephone and (b) direct access to the exchange line from the
executives' extension telephones, dispensing with the need for secretarial
The intermediate (secretary's) telephone is simple to operate. It incorporates four
press buttons. The outer two, designated RING 1
and RING 2, are used for selective calling of extensions; the inner two, designated EXCH and EXTN,
are for switching functions. Exchange calls are answered by
pressing the EXCH button and
then offered to either extension
as required by first pressing the
EXTN button, followed by the appropriate RING button.
The exchange call is held automatically while the call is offered, and is transferred automatically
when the handset at the intermediate telephone is replaced.
Should the extension user not
accept the call, the EXCH button is re-pressed to permit return to the exchange line.
If the intermediate instrument is
temporarily unattended, an exchange call can be signalled at either extension on manual release
of the bell on/off pressbutton, and
the call accepted. During exchange calls at either extension, the 'engaged' lamp glows at the intermediate.
Intermediate to exchange with buzzer available for extension calls. Extensions cannot overhear.
Intermediate to extension with bell
across the exchange line. The secretary at the intermediate can ring and speak to either extension. Information call from intermediate
to extension while the exchange line is held. The exchange caller cannot overhear. Transfer of exchange calls from
the holding circuit to the wanted
Direct access to the exchange line at both extensions when all buttons at the intermediate are normal.
Secrecy on extension-to-exchange calls by simple strap adjustment. If non-secret working is imposed, the intermediate (secretary) may
join the conversation. Intermediate recall permits either extension user to recall the intermediate during an exchange call.
The 'engaged' lamp glows at the intermediate when either extension engages the exchange line.
Selective calling from the intermediate to either extension, if there are two.
Direct calling from either extension
to the intermediate.
Extension-to-exchange calls made direct without switching through
at the intermediate.
Exchange calls received at either
extension when the intermediate is unattended.
Power and cabling
A 6V battery or power unit is required at the intermediate instrument.
Cable supplied as required; 6-way
for installations with one extension and 7-way for installations with two extensions.
A MAIN TELEPHONE AND PARALLEL EXTENSIONS
HAVING DIRECT EXCHANGE
This is a simple system, suitable when
one exchange line is required to serve several internal extensions.
All exchange calls are answered at the
main telephone and passed to the required extension.
The main telephone is available with
either 4 or 8 press buttons and an AC bell.
The extension telephones incorporate a
AN INTERMEDIATE TELEPHONE WITH EXTENSION SWITCHING
AND AN EXTERNAL EXTENSION
This Plan system is suitable for
users with one exchange line serving two locations some distance apart; for example, a shop
and associated warehouse. It consists of an intermediate and an external-extension telephone connected by a 2-wire line. Bothway
signalling between intermediate and extension is by in-built transistor ringing units. The loop resistance between the external extension and tie main
exchange can be up to 1000 ohms.
At the intermediate telephone, four locking press buttons are provided for the following switching and signalling functions:-
Intermediate to exchange with a.c.
buzzer across the extension line.
Extension cannot overhear.
Intermediate to extension with a.c.
bell across the exchange line.
Information call with the exchange
line held. The exchange caller cannot overhear.
Extension to exchange direct. Calls
can be secret or non-secret from the intermediate.
Incoming exchange calls are normally answered at the intermediate,
and are held when offered to the extension. The call is transferred
automatically when the EXTN-EXCH button at the intermediate telephone is operated. Should the extension user not accept the call,
the intermediate user may return to the exchange line by pressing the EXCH button. During extension-to-exchange calls the 'engaged'
lamp glows at the intermediate.
Intermediate to exchange with a.c. buzzer across the extension line.
The extension cannot overhear.
Intermediate to extension with a.c. bell across the exchange line.
Information call from intermediate
to extension, with the exchange line held.
Extension to exchange direct When
the extension handset is in position, the intermediate-telephone bell is connected across the exchange line to signal incoming exchange or extension calls.
Transfer of exchange calls from
the intermediate to the extension.
Return to the exchange line if not required to transfer an exchange call after an information call. Secrecy on extension-to-exchange
calls by strapping adjustments. Engaged lamp signal given at the intermediate during extension-to-exchange calls.
Ring extension condition applied by over-pressing the EXTN switching button, thus avoiding the need for a separate button.
PBX recall or transfer button provided if required.
Power and cabling
A 6V battery or power unit is required at the intermediate and extension instruments.
A 2-wire cable is necessary between the intermediate and extension telephone.
Taken from the Plessey Telephone Catalogue (1971)