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. 

1.  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.

2.  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 value.

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 coin handling components. 

Nickel-cadmium secondary battery No 35