TELEPHONE No. N2020, N2021 & N2022


The Plesseyphone meets the most up to date requirements for a telephone of compact proportions, advanced design and economic cost.  Built on radically new principles with new and improved components, it combines simplified construction with functional elegance, backed by high-quality transmission based on CCITT recommendations.

The new telephone is compatible with all types of automatic and C.B. exchanges, and can be used as a main or extension instrument, fitted with optional automatic line regulator, shared-service or recall button as required.

Standard provisions include several usually offered as extras: ringer-volume control, carrying facility (rear recess) and tropical finish.  Alternative forms of dial numbering are available to suit customer requirements.

External Features
The broad, lower profile and angular corner treatment of the telephone is in keeping with the more mature current design trends.  The traditional handset locating ‘horns’ have been eliminated.

The symmetrical lightweight handset- conforming dimensionally with CCITT recommendations - is located by mouthpiece and ear cap into side recesses in the case, which contain the bar-type cradle switch plungers.  The base, which is fitted with non-slip feet, is formed as a low plinth; this styling feature is emphasised by suitable colour contrast as shown in the table below.  Black telephones are available if preferred.

The transparent dial fingerplate, of tough acrylic, is larger than hitherto.  Finger holes are flared for easy finger engagement and reduced slipping.

Handset cord entry is front-central for convenience of both right and left handed users and to prevent cord fouling of the cradle plungers during conversation with consequent loss of the established call.

The thin, extra lightweight handset cord is the coiled extensible type as shown in the illustration below.

The rear recess in the case permits comfortable carrying of the instrument.

Scheme 1 Scheme 2 Scheme 3
Case and handset hemp beige lava green silver grey
Base loam brown ilex green concord blue
Cords elephant grey ilex green concord blue
Desk block dusk grey dusk grey dusk grey


The case, base and handset mouldings are of ABS (acrylonitrile-butadienestyrene), an extremely tough material of excellent surface finish and uniform, lightfast pigmentation, with good resistance to staining.

Integral features of the case moulding include the rear carrying recess, a housing for the dial mechanism, and shape-stabilization ribs.  The handset is a similarly stabilized assembly of two parts only, which snap together - holding transmitter and receiver captive - and are further secured by screws.  The telephone base is provided with lugs for spring-clip attachment of the ringer.  Ringer sound is transmitted through slots in the base and in the carrying recess; these slots also provide ventilation but are sufficiently narrow to prevent entry of insects.  Attachment of base to case is by engagement of lugs at the rear and a single screw fixing at the front.

Case, handset and line-block mouldings incorporate integral cord grips with staggered arrangements of pegs which remove all strain from the conductors and terminations within.  The line-block moulding is in one piece, with a snap on cover; four connectors are fitted for plug-in line-cord connection, and screw-termination of incoming cable.

Circuit board
All elements except the ringer are mounted on, or directly attached to, this board.  The printed circuit is die stamped for maximum robustness and is protectively coated.  The board plugs on to the underside of the dial - no dial cord is necessary - and is secured to the case by three screws, forming a strong, shock-resistant assembly in which the components, notably the cradle plungers, are closely controlled in position relative to the case.  The board also completes a virtually dustproof enclosure for the dial mechanism.

Micro-switches are employed instead of conventional cradle switch springsets, these being directly attached to the board, as are fulcrum brackets for the transverse operating bar.  Flat extensions on the end of the bar form the cradle switch plungers.  Contact pressures of 30gm minimum are realized.

Capacitors are single-unit metallised film types in moulded cases, giving complete protection from tropical conditions.  The induction coil is a high efficiency miniature type.

Terminations are made by push-on edge connectors.  Their designations are marked on the lower side of the board.

Regulator (optional)
The regulator operates on conventional principles; the magnitude of the d.c. line current - an inverse function of line loss - determines the resistance of a non-linear element, which in turn fixes the transmission efficiency.  Users thereby experience reasonably identical speech levels irrespective of exchange distance.

Maximum losses introduced, i.e. at zero line, are 4dB for sending and 6dB for receiving.  Both losses decrease progressively with increasing line length, and become negligible at the limiting loop resistance.

Dial mechanism
The standard speed is 10 + 1 p.p.s. and the nominal make/break ratio 1 : 2 with limits of 64% to 70% break.  An increased speed up to 20 + 2 p.p.s. and/or other make/break ratios can be provided to order.  The lost-motion period (basic I.D.P.) is 240m sec minimum.

The mechanism is secured in the case housing by three screws.

This is a new unicoil, double-acting design of high sensitivity, with a unique moving magnet/armature system.  A robust torsion suspension replaces the usual pivots, eliminating the problem of wear. Movement, gongs and volume control lever form a single assembly when mounted on the base.  The minimum volume is sufficient to ensure that incoming calls are detected.

An immersed electrode carbon type of improved design is fitted.  Diaphragm parameters and dimensions of adjacent cavities are chosen for optimum frequency response and sensitivity.

This is of capsule type, featuring a single-coil sealed driving unit with all elements arranged coaxially, resulting in a fully enclosed, completely self shielding magnetic circuit.  Maximum performance is ensured by a composite diaphragm system whose three elements - armature, suspension disc and sound radiator - each have a single function, avoiding compromise in design.  Units are individually adjusted in manufacture for optimum sensitivity and frequency response.

Ordering Information
When ordering, please quote ‘PLESSEYPHONE’ and appropriate codes and information from the list below:-

Equipment Item Code No.
Auto telephone N2020
Auto telephone with button for shared service/recall N2021
C.B. telephone N1520
C.B. telephone with button for recall N1521

All types are available in the following variants:-
Code suffix A - for line loops up to 1000 ohms and with automatic performance regulator provided.
Code suffix B - for line loops up to 1000 ohms with no automatic performance regulator provided.
Code suffix C - for line loops up to 600 ohms.

Colour scheme
3-way (standard) or 4-way line cord
Dial speed and ratio
Dial numerals

Taken from the Plessey Telecommunications Publication No. 7074/2 (1/69)


Telephone Rental models

Ericsson Code Colour
N2020C21T Hemp Beige / Loam Brown
N2020C22T Lave Green / Ilex Green
N2020C23T Silver Grey / Concorde Blue
N2020C24T Black


Table telephone for use on auto exchange lines.

N2020 - Dial, Auto - introduced 1968.
click here for blueprint

N2021 - Dial, Auto - with button for shared service/recall.

N2022 - Press button dialling - MF
click here for blueprint

This model has all components mounted on a circuit board which is fixed to the top casing.

The picture below shows an LD (dial) version of which the manufactures code is unknown.

Plessey Plesseyphone

Very much a product of the 1970s, the Plesseyphone was a brave attempt by the Beeston plant to produce a functional dial telephone at a more realistic cost than the (expensive) BPO specification Telephone 706 and 746.  A brave attempt but not a wholly successful one, for the Plesseyphone was not a success; it was not liked by customers and in the end, vast quantities were dumped on the surplus market at low cost.

The whole telephone was a masterpiece of value engineering (in other words, cost reduction).  The dial was integral with the case of the telephone, with the springsets and mechanism moulded under the case and the dial numerals screen printed on top of the case.  A new design of one-piece terminal block was used, with the cover hinged to the base and held closed by a single screw.

This telephone was also tropicalised by default which, according to documentation, was achieved by the slots in the base that "give adequate ventilation to the interior, while admitting sufficient light to deter any insect from setting up home in the instrument".  The copper layer of the PCB was die-stamped which apparently "provides a robust conducting path, free from lift and hairline cracks and - of special significance under tropical conditions - corrosive residues".

The BPO also tested this phone, in 1976, but decided not to use it in their portfolio as the phone was difficult to re-assemble, the receiver wires could be pinched between the two handset mouldings, the self tapping screws in the handset/case tended to become cross threaded and the phone did not use similar parts to the ranges the BPO already used.

Silver grey, lava green, hemp beige, blue, ivory and black.

Very few (PAX systems). The telephone was flimsy and lightweight, prone to skidding across the desk.  Line and handset cords were skimpy and the whole phone had a cheap look and feel to it.  The handset squeaked under hand pressure and the dial numerals screen-printed onto the telephone case were soon erased by people who used pencils to dial.  The kindest thing to say is that the colour options were imaginative and attractive.

Telephone Rentals Model

Telephone Rentals also used this model, but the circuit is very basic as it would have only been used on internal telephone systems.  The regulator and induction coil have been omitted.  Probably the Type C variant.


Diagram N118093

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Last revised: October 30, 2021