TELEPHONE No. 722, 1/722 & 8722

TI - Description of Trimphone
How to wire your Telephone No. 722 to make it work on PST
Click here for history of the development of the Trimphone
Sales Circular 342/65 - Telephone No. 712
Sales Circular 121/68 - Telephone No. 722
Marketing Brochure PH1950
Marketing Brochure PH1667
Extract from Hansard
General fault finding on your phone
Other colours - Public survey

Circuit Diagram - N822 and SA/SAW 10171.

Drawing - 92772, 92272C and 92272F.

Specification - S1008.

Tone Ringer No. 5A Circuit Diagram - N692.

Designed and manufactured by STC, their Deltaphone.

The Trimphone started life in 1964 as the Telephone No. 712 Trimphone. The (then) modern design incorporated the novel feature of dial illumination, tone calling and a unique handset.  The initial four letters of the name Trimphone stand for Tone Ringer Illuminated Model.  The Trimphone was designed by Martyn Rowlands.

The handset was coded 'Handset No. 8' and featured smaller transducers (Inset Receiver No. 3T and Transmitter No. 15) mounted adjacent to one another in the earpiece cavity.  The transmitter being coupled to the mouthpiece by an acoustic horn.  The transmission circuitry was based on that of the Telephone No. 706.

The Trimphone was the first in the BPO range to use a tone caller which warbled at around 2000Hz modulated by ringing current.  The volume of the ringer gradually built up over the first few cycles of ringing current.  There is a volume control in the base of the telephone with LOUD, MEDIUM and SOFT settings. Some people were able to mimic the sound of the tone ringer by simultaneously whistling and wobbling their lips... a vulgar habit which should be frowned upon.

Production of the new telephone commenced in 1965 by STC, and an initial quantity of 1000 was offered to customers on a selective trial basis.  It then became freely available, at extra rental cost, with a choice of three two-tone colour schemes: Grey-White, Grey-Green and two-tone Blue.  The hollow handset led to some embarrassing results when customers attempted to cover the mouthpiece by hand in order to make a confidential aside - the sound was still transmitted inside the handset!

Another problem with the dial version of the Trimphone was its light weight, 0.8kg compared with 1.4kg for the 700-type and 2.6kg for the 300-type telephone.  This led to the complaint that on slippery surfaces the telephone turned and slid whilst dialling.

Standard Colours

Blue Green Grey

There was also some concern about the luminescent dial which glowed green in the dark.  Although the radioactivity was equivalent only to that given off by a wristwatch it was felt wise to withdraw this facility as public concern over radioactivity grew.

An improved version, the Telephone No. 722, was introduced in 1971.  This had the same outward appearance as the Telephone No. 712, but had the improved transmission circuitry of the telephone No. 746.

Then came the telephone No. 1/722 and this was fitted with a Telephone-Unit D93050, whilst the Telephone No. 722 was fitted with a Telephone-Unit DD92744.

The next variant was the Telephone No. 2/722 which used a Tone Ringer No. 8A in place of the Tone Ringer No. 5A.

From 1973 some models were fitted a Regulator No. 7A (blue plastic-covered component).  This was connected across the transmitter on terminals 3 and 10 and was an attempt to protect the transmitter from the effects of excessively high current on short lines, thus reducing faulty transmitters.

Introduced in 1982, the Telephone No. 722 was replaced by the Telephone No. 8722, which was fitted with high impedance bell coils (4000 ohms) and a Cord Conn No. 4/504 3000mm.

These telephones were made in the STC factory in Larne, Northern Ireland (TCH), but Trimphones were also manufactured by GEC-AEI Telecommunications in Newton Aycliffe, County Durham (GEN), and at the GPO Factory in Cwmcarn, Wales (FWR). 

The First Keypad Trimphones
The first keypad version of the Trimphone appeared in 1977 - somewhat delayed by the problem of packaging the signalling electronics into the small volume of the Trimphone.  The problem was alleviated by marginally increasing the height of the case compared to the dial version.  The first design of keypad Trimphone to achieve large-scale production was the SC version (Telephone No. 766); this design incorporates relays, but no batteries are needed. Subsequent designs have eased the packaging problems further by eliminating the relays and introducing transistor pulsing.  An MF4 (Touch-tone) design had to await the development of an integrated circuit to replace the bulky coils and capacitors otherwise needed. This was introduced in 1979 (Telephone No. 786).

The Deltaphone
The next incarnation of the Trimphone was the Deltaphone.
By 1980 Trimphones had been around for some 15 years and needed revamping for the new era of competition.  STC gave Trimphones this new lease of life by renaming the dial version as the 'Deltaphone' and the MF version as the 'Deltaphone Deluxe'.  The transformation was tastefully completed by cladding these unfortunate instruments in leather.  Mid-brown for the Deltaphone and a choice of Red or Green for the Deluxe model.

The Phoenixphone
The next stage in the life of the Trimphone was the Phoenix Phone Campaign.  This final incarnation (or rather reincarnation) of the Trimphone was a collection of alternative colour range Trimphones.  These were the first part of a series of telephones known as 'Phoenixphones' and this first set was called the 'Snowdon Collection' and appeared around 1982 .  Six two-tone colour combinations were chosen 'by leading design consultants' and were available in both dial and press-button versions.   It is believed that Lord Snowdon had a part in these colour choices...

Click here for larger colour pictures of the Snowden Range

   BODY    HANDSET           
   Olive Green   Beige
   Dark Orange   Orange
   Dark Red   Red
   Black   Grey
   Light Green   Mid Green
   Brown   Cream

The telephones were refurbished (or rose from the ashes! hence the Phoenix association?), called the Telephone No. 8722G and were fitted with the then new Plug and Socket cords (Cord Connecting No. 4/504 Light Grey).  No rental option was available only outright sale at £35 for the dial version and £46 for the Press-button model (both prices include VAT).  The sales literature pictured a press-button Phoenixphone languishing on a rush table mat with a scallop shell (ash tray?), flip-up photo album and dried plant.  Was this taken in Lord Snowdon's front hallway?

The Trimphone has been with us in one form or another for some thirty years now, and although it is unusual to see one in use these days there are still quite a few around... in fact Trimphones are still seen for sale either new or refurbished in some telephone shops at the time of writing! (1995).

Despite the complaints in the early days about it slipping whilst dialling the Trimphone has become one of the icons of British telephone design and has surely earned the right to be in any discerning telephone collection!

The Phoenixphone Cont'd
As mentioned previously in the notes for the Trimphone there was a series of telephones known as Phoenixphones which were available only for an outright sale from telephone shops.  The first Phoenixphones were standard Trimphones (Telephone No. 8722G's) already described which became available in 1981.

There is rumour of two other issues - the second in the series were Telephone No. 8756's (self contained (SC) versions of the Telephone No. 746) which were available in the following colours: Pampas/Primrose/Lime/Mushroom/White.

A final third 'Phoenixphone' was the Telephone No. 8706F available in Grey or Ivory.

All Phoenixphones had high impedance bells and Plug & Socket style cords.  It is believed that they were made primarily from refurbished parts, although the cases must have been new as they were in colours not previously available.

These Trimphone notes were gratefully obtained from:-
IBTEJ V5 p263, IPOEEJ V58 p8, IPOEEJ V74 p239, BPO sales literature and Rob Grant.

trimring.gif (30154 bytes)
Click to hear a Trimphone warbler

3 Internal
Issue 3, Jan 1982

(This TI incorporates the information contained in B1016 - 1971)

This instruction describes the Telephone 2/722 which is a telephone having the distinctive features of a small and elegant design, a self-illuminated dial (in condition of darkness) and a tone ringer with a volume control, instead of a magneto bell.  It is popularly called the Trimphone, derived from "Tone Ringing Illuminated Model" and is available to be a table model or can be wall mounted using the Kit No. 422A.

Earlier versions of the telephone, designated Telephone No. 712 and Telephone No's 712 Mark II, 1/722 and 1/722 MOD were subjected to field trials and telephone 2/722 is the final design, incorporating several improvements on the initial designs.  The telephone, as its code implies, is of the 700 type, i.e. its transmission components and circuitry make it suitable for use on local lines of up to a maximum of 10 dB at 1600Hz.

The Telephone 2/722 is an addition to the range of 700 type telephones and does not supersede Telephone Nos. 706 and 746.

The handset cord entry is at the left-hand side of the base; the line cord is of the extensible type and emerges from the rear.  There is no 'off-rest' position for the Handset.  The transparent plastic cradle-bar serves both as a gravity-switch plunger and carrying handle. The edge-operated volume control knob for the tone ringer protrudes slightly from the telephone base near the front of the set on the right-hand side.

The telephone, with handset on, is 216 mm long, 108 mm wide and 114 mm high overall, weighs approximately 0.9 kg and is available in three two-tone colour combinations, light blue/dark blue, middle grey/olive green and off-white/light grey, the first colour referring to the telephone cover and the second to the handset.

The telephone cover is secured to the assembly by two lugs on the front skirt which engage in the recess in the leading edge of the base and by a single captive nylon screw on top of the cover in the handset recess.  The screw engages in a tapped pillar fixed to the telephone unit. The cradle-bar and gravity-switch operating lever are integral with the cover.  The telephone circuitry is in the on-hook condition with the cover removed, and because of this a gravity-switch latch is unnecessary.

The base of the telephone is of moulded plastic with a common colour of dark grey.  The front leading edge is normally fitted with a dummy insert which can be removed to fit a press button (Switch No. 13A-2).

Moulded ribs support the tone ringer and transmission component boards.  The four feet are the same as those of the Telephone No. 746 (Part No 2/DBU/259).

The dial (Dial Automatic No. 30) has a standard trigger type pulsing mechanism but the number plate is illuminated from beneath by a C-shaped fluorescent tube containing a radioactive gas (Tritium).  The level of illumination is sufficient to locate the telephone in the dark and to read the dial characters.  The finger plate is of slightly tinted polycarbonate.  The dial face is orientated in an anticlockwise direction by one finger hole distance (rotationally about 25 degrees) compared with the standard No. 21 type dial fitting.  This orientates the finger stop by an equal amount and alleviates the tendency of the complete telephone to slide sideways to the left when the dial is operated.

The dial mounting is a four legged semi-rigid plastic moulding which straddles the tone ringer.  The two rear feet wedge beneath the edge of the telephone unit board and the front ones are sprung into the front corners of the base.  The dial is a wedge fit into the annular portion of the mounting which includes an integral dust cover.  When the telephone is assembled the dial is held firmly in place by the flanged rim of its body pressing against the underside of the cover, the hole in which only allows the finger plate to pass through.  The dial cord has spade-tags at the dial end and ring-tags connected to terminals on the telephone unit at the other.

Internal view showing optional press button switch installed under dial on the front edge of the base

The printed wiring board (Telephone-Unit D93050), which uses transmission components similar to those of the Telephone No. 746, rests on the edges of the base at the rear of the instrument and is held in place at the front by two moulded lugs on the base; a single screw passes upwards through the base into the bottom of the cover-supporting pillar fixed to the board.  All the components of the transmission and regulator circuits, the gravity micro-switch and telephone terminals are mounted on the upper side of the board.  The circuit diagram of the Telephone No. 2/722 (N822) is the same as diagram N846 (Telephone No. 746) except a tone ringer, instead of a bell, is connected between T4 and T16.

The tone ringer contains a transistor oscillator powered by rectified ringing current which actuates a transducer similar to a Receiver Inset No. 3T, causing it to emit a warble-tone.  The intensity of the tone can be varied by a four position control knob located at the front right-hand side of the base of the telephone.  The telephone No. 2/722 includes a 2 terminal tone ringer (No. 8A) to overcome some of the installation and service difficulties experienced with previous Trimphones.  The new tone ringer includes a delay circuit to prevent spurious pulses of tone and dispenses with the need for a series Thermistor on exclusive connections.

All the components are mounted on a printed wiring board with flying leads for connection to the terminals of the telephone unit.  The board is fixed by a single screw passing through it into a raise boss on the base of the set.

The four settings of the control knob are:-

OFF - No sound
SOFT - Soft warble tone of constant intensity
> - Soft warble tone gradually building up to an intensity similar to that obtained in the loud position
LOUD - Loud warble-tone of constant intensity

The OFF position is not normally available to customers with a single telephone.  However, when the telephone is used as an extension requiring a 'Bell Off' condition an engineering officer can achieve this by removing the stop-screw from the mould boss alongside the 7 o'clock position of the volume control switch wafer and parking it in a similar boss at the 5 o'clock position.  The picture below illustrates this operation.

Early issues of the Telephone No. 722 were fitted with a Tone-ringer No. 2B in which the "Tone ringer off" facility was slightly different.  Trimphones with Tone Ringers No. 2B should not be re-issued, but should be returned to Supplies Division as old stock.
Click here for instructions on the on/off screw

The handset is coded Handset No. 10A-1.   The novel design of the handset uses the small transducers - Receiver-inset No. 3T and Transmitter-inset No. 15 - designed originally for the Headset No 1.  Both transducers are fitted inside the handset earpiece and an acoustic horn within the handset handle couples the mouth-piece to the transmitter.  The complete handset, with cord, weights only 156 grams and can be held comfortably between finger and thumb.

A screw at the end of the handset holds the earpiece in place and its removal exposes both transducers.

The transmitter-inset is held in the body of the handset by a spring clip, the open ends of which go a short distance into the handle.  The clip clamps the inset against the throat of the acoustic horn.  The horn is removable to facilitate cord changing.

The receiver-inset is held in position against a grommet in the earpiece only when the handset is assembled; the curved tail of the transmitter-retaining clip then presses on the back of the receiver inset.

When reassembling the handset, care must be taken to avoid trapping the cord conductors check also that the spring clip will not short the receiver terminals.

This consists of a plastic wedge shaped box (wall bracket), handset clip, cradle bar and a longer case retaining screw.  To install, fit the bracket to the wall using the three securing holes.
The following modifications will have to be carried out to the telephone instrument:-

  1. Remove the four rubber feet on the underside of the instrument.

  2. Remove the existing line cord.  The exchange line is then connected to the instrument via the hard wiring.

  3. The existing cradle bar may need to be replaced if it differs from the new cradle bar found in the Kit No. 422A.  The exchange line cable should be brought into the wall bracket via the bottom, under the sloped back panel that contains the fitting instructions and behind the cable retainer which is situated as part of the top right-hand telephone support hook.

Once the cable has been routed as described the cable should be cut to length i.e. to the bottom edge of the wall bracket.  Then strip the outer sheathing to approximately 50mm below the support hook.  Hang the instrument on the two support hooks of the wall bracket and clip the lower half into place to secure.  Terminate the exchange line cable.  Replace the telephone cover and place the handset clip in position then secure by using the longer case retaining screw.

Two add-on units are available, a single changeover press-button and a single changeover auxiliary gravity microswitch.  The press-button (Switch No. 13A-2) is fitted in place of the dummy insert on the front edge of the base.  The wide transparent operating lever projects beyond the cover.  The auxiliary gravity microswitch (Switch No. 19A-1) is attached to the normal gravity-switch by a two-part clamping bracket having a screw which passes through a hole in the normal gravity-switch moulding.  The gravity-switch plunger lever operates both microswitches simultaneously.

Diagram N822 shows uses for both items. The add-on units used are the same as for the Telephone No. 1/722 and are listed below with their Telephone No. 746 equivalents.

Switch No. 5A-3 - Switch No. 13A-2
Part 1/DSP/1233 & 1/DSP/1252 & 1/DSP/1256 - Switch No. 19A-1
Switch No. 5A-3 & Part 3/DBU/264 - Provided by the off position of button, the tone ringer (bell on/off)
Buzzer No. 32A-1 or 32C-1 - Buzzer No. 20B (fitted adjacent to the telephone block terminal)

Because of its small size the Telephone 2/722 cannot be fitted with many of the add-on units available to other 700-type telephones and its use is, therefore, restricted to the following applications:-

DELs exclusive and shared service.
PBX extensions - Two wire and four wire.
Extension Plan 1 main with one extension only - exclusive.
Extension Plan 1 extension with separate d.c. bell - exclusive and shared service.
*Extension Plan 1A main and/or extensions exclusive and shared service.
*Extension Plan 4 one portable telephone exclusive and shared service.
*Extension Plan 4 two or more portable telephones - exclusive and shared service.
Extension Plan 105 and 105A external extension - exclusive.
Extension Plan 107 and 107A internal extension with separate buzzer exclusive.
Extension Plan 107 and 107A external extensions - exclusive.
Extension Plan 108 and 108A main telephone - exclusive.
NOTE: Those Plan numbers marked with a "*" have now been superseded by Phone Socket (Plug and Socket).

For the time being stocks of Telephone No. 1/722 MOD may also be issued against Trimphone requisitions and these should be used to provide service on direct exchange lines and direct PBX extensions only; plan extensions should be provided by means of Telephones 2/722.

Four-way and 6-way line cords are available. Four and five way plugs with cords for Plan 4 arrangements although the Plan 4 has now been replaced by Phone Socket which uses a four way plug as part of a Cord Connecting No. 4/504.

4 way Plug 420 grey - 4A (grey cord) - 4B (blue cord) - 4C (green cord) Cord Connecting 4/504 (length 3000 m)
Note:- Suffix letter indicates colour of cord.
5 way Plug 505 grey - 1A (grey cord) -1B (blue cord) -1C (green cord)

Certain plan extensions, e.g. exclusive Plan 107 internal, require a buzzer to be associated with the extension instrument.  In the case of the Telephone Nos.706 or 746 this buzzer is located within the telephone and a change of line cord from 4-way to 5-way becomes necessary.  When Telephone No. 2/722 is used for these applications, the buzzer must be fitted external to the telephone, and the need to change the standard 4-way line cord on this account does not therefore arise.

The table below lists the replacement parts which are available.



Telephone unit

Telephone-unit D93050

Tone ringer

Tone-ringer No. 8A

Cover (includes gravity-switch plunger)  

Cover No. 81A, colour


Handset No. 10A-1


Receiver-inset No. 3T


Transmitter-inset No. 15


Dial, Automatic, No. 30

Dial finger-plate

Part 3/DPL/2145

Label protector

Part 1/DPR/6

Number ring

Label No. 483

Line cord (helical)

Cord, Inst, No. 4/108 AX, 54 in, colour

Line cord (straight)

Cord, Inst, No. 4/110 AX........ colour

Line cord (for Phone Socket)

Cord Connecting No. 4/504

Handset cord

Cord, Inst, No. 4/109 AX, 10 in, colour

Dial cord

Cord, Inst, No. 5/43L


A new item coded Regulator No. 7A has been developed to extend the life of the transmitter inset.  This device should now be incorporated in the Trimphones but also are available to be added separately by field staff if required.  This device is fitted across the transmitter terminals inside the telephone and is a blue plastic covered cylindrical coil just over an inch long with two fly leads.

Additional information

Tele 722FMk 1yyy6/66 Obsolete 4/82
Tele 722LMk 1yyy6/66 Obsolete 4/82
Tele 1/722FMk 1yyy11/69 
 Mk 2yyy11/69Improved circuit design
Tele 1/722LMk 1yyy11/69 
 Mk 2yyy11/69Improved circuit design
Tele 2/722Mk 1yyy6/71 Uses Dial No. 53
 Mk 2yyy9/73Regulator No. 7A fitted
Tele 2/722AMk 1yyy5/72Uses Dial No. 53
Tele 2/722FMk 1yyy 

Uses Tone Ringer No.8A

  Mk 2 y y y    
Tele 8722GMk 1yyy 9/82

Plug & Socket version

The 722F was superseded by the 2/722F in 1982.
The 1/722F, 2/722 (field trial) and 2/722A were made obsolete in 1983.

Additional Picture
Dated 1967

Internal picture - There is an auxiliary micro switch fixed the gravity switch hooks with two screws

Internal picture - showing the gravity switch hook mechanism inside the case
An additional switch is also fitted in the front of the base

This pictures shows a Receiver, Watch fitted the rear.
This was probably a prototype which was never adopted.


Viscount Hall (Chairman, Post Office) and Kenneth Corfield (Chairman, STC)
with an oversized Trimphone, 1970.

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Last revised: December 17, 2022