CT24 CT25

Plessey Push Button Payphone

Coin Telephone 24A - Payphone 600 - for PCO

Coin Telephone 25A - Payphone 500 - For renters

Green Circle Approval S/100/3/G/452077 ?

To get these payphones out of "999 calls" only mode you need a TESTER 297A, I now have one, as Id like to work out what the protocol is.

The new payphone was being developed under contract by Plessey Telecommunications Ltd. and was scheduled to enter public service in 1983.

The CT24/25 provided all of the facilities of the CT22, and employed a single all-electromagnetic coin checker

for all coin values from 2p to £1.

Payphone 500 LCD display

note Least significant digit has a segment broken and "minimum" legend is not displaying

Like the CT22, a 4-digit display of available credit is provided, and charges are

debited indirectly in 1p or 2p steps. However, the display on the CT24/25 is much more comprehensive: at the beginning of a call it shows the minimum fee needed, and at the end it indicates how much will be refunded.

The CT24/25 payphone receives periodic meter pulses from the local exchange, it is connected to ordinary (ORD) gradings and thus the call charges must be based on the same charge-unit durations as those for ordinary lines.

The call charges are in 2 parts: a minimum fee, and a unit fee. In the BT payphones, the minimum fee covers the first metered unit, and subsequent units each cost one unit fee. The ability to charge extra for the first unit is extremely useful because it enables tariffs to be tuned more accurately to the true costs of providing a PCO service ; and where a payphone is rented, it allows the renter to obtain a realistic return on his investment. Once a call is under way, there are 2 basic methods of deducting the charges from the credit established. In the simpler approach, called direct debiting, the minimum fee is deducted on receipt of the first meter signal and one unit fee is deducted on each subsequent signal. Note that the unit fee need not be a multiple of any coin value, nor even of one penny; but in practice BT have adopted 1p as the smallest adjustment. A more complex debiting system, used in the CT24/25 payphones, is called indirect debiting . After the

receipt of the second meter signal, the payphone debits the call charges in 1p or 2p steps at intervals calculated by dividing the measured meter period by the unit fee. This enables the unit fee to be set to a value that is not in whole pence without

causing confusing rounding errors on the credit display, and gives the customer the advantage of being able to extend his

call by increments that are smaller than the unit fee. Another feature of the call-charging system is the ability of

the payphone to do its own call timing under certain conditions. Because of the usual pattern of usage of payphones, the

BT tariffs normally allow a shorter time per unit fee in the local-call cheap-rate period than is applied to ordinary

telephones. The payphone applies its own timing when the received meter-pulse interval exceeds a specific va lue.

Owing to the methods used in the generation of periodic meter pulses, the meter interval can vary from pulse to pulse

on a given call ; further, there may be a change of tariff period in the middle of a call. The payphone is required to take these

variations into account as well as applying indirect debiting and internal timing of local calls. In addition, great care has to

be taken not to exceed the published charges, whilst not systematically under-charging. The result is a complex debiting

procedure which constitutes a very significant proportion of the control software.

The CT24/25 is entirely line-powered, using a CMOS microprocessor (RCA 1802 family) and light but robust coinhandling


Nickel-cadmium secondary battery No 35

A nickel-cadmium secondary battery maintains the data RAM and supplies current peaks for coin-handling as well as enabling the payphone to operate for short periods with the line disconnected (such as, during cashing and refunding at the end of a call). The battery is trickle-charged from the line during idle periods but receives most of its charge while the line is seized.

CT24 AND CT25 Compared

The Public Phone box or PCO version (designated CT24 or payphone 600), and the renters' model (CT25 or payphone 500) have been designed with as many common parts as possible. Externally, the only visible differences lie in the size of the cash compartment (which, in both versions, is separate from the mechanism housing for added security) and in the more robust handset of the CT24; but the CT24 has been made very much tougher than its sister by the use of stronger materials and locking arrangements. The internal mechanisms of the payphones use identical major assemblies, with a few small components (such as, a relay to provide a renters' local cash-box-full alarm) being fitted only in the CT25 or being optional. In addition, the CT24 will have ceramic-packaged integrated circuits to cope with the greater extremes of temperature and humidity, though the printed-wiring boards will be otherwise identical.

£40 million divided by 77,000 = £519 each

payphones for New Zealand

Bob has four of pages on his site about these.....

1 Coin Telephone No. 24A a 10 button trial ? blue Telephone 2 with Handset No. 3A Black

2 BT PAYHONE 600 a stainless 12 button model for renting Handset No. 3A

3 COIN TELEPHONE No. 25A yellow 10 button for renters ? Handset No. 16

4 BT PAYHONE 500 12 button keypad in available in stone/brown or yellow/black or brown Handset No. 16


Renter Payphones by John Hough, Product Manager for Renters' Payphones.

The present Pay-on-Answer (POA) payphone has served us for many years but is now becoming technically out-dated and also has major shortcomings for the 1980s, including tariff inflexibility, no access to the most distant IDD bands, vulnerability to fraud and poor reliability. Previous FOCUS articles have covered the new Public Call Offices (PCO) and Cardphones; this article describes the new electronic products that are being developed now to replace the present Renters' Coinboxes (RCB). These new microprocessor controlled products employ the latest technology and offer greatly improved facilities for our customers. They will also help to open up new markets.

The aim is to replace all POA RCB's with new payphones by 1989. To this end, three new products are currently being developed or planned. The new payphone will work to Subscribers' Private Meter relay sets at the exchange, not Coin and Fee Check equipment as the POA payphones do.

They will all accept a wide range of coins, allow self-dialled local, STD and IDD calls, refund unused coins and be attractively styled. Beyond these similarities, each product is designed to meet the differing needs of different market sectors.

BT Nursery rhyme phone trollies at westminster Children's hospital