Pay on Answer (POA)

BOXES COIN COLLECTING No's 700 & 705 & 735

When subscriber trunk dialling (STD) was introduced in 1958 there arose the need for a coinbox system which could deal with the new periodic metering for both local and trunk calls.Before STD a local call had a fixed fee however long it was. In order to keep the coinbox mechanism as simple as possible so that capital and maintenance costs might be reduced, a post-payment system (known as pay-on-answer) was adopted; coins were cashed on insertion and not held in suspense. A complex piece of apparatus at the local telephone exchange, known as the coin-and-fee check (C and FC) relay-set, compares the meter signals from the call-charging equipment with the coin signals received from the coinbox. The C and FC unit controls the opening of the coinslots, gives the customer audible warning when his money has run out, and cuts off the call on expiry of a short period of grace.

Red Coin Collecting Box 735

Shane's video of a POA Phone in action

To the GPO, the POA coinbox system had many limitations, of which the 2 most affecting profitability were the lack of flexibility in setting tariffs, and the poor reliability of the system. The user difficulties mentioned above also affected profitability by discouraging people from making high-tariff calls. The basic difficulty over tariff-setting was that the unit fee had to be equal to a single-coin value ; and whilst fine adjustments can be made by altering the timing of the meter signals in the network, the unit fee is a significant parameter because a very large proportion of calls last for only one meter period. In times of high inflation, unprofitable tariffs were retained 
in deference to external pressure on GPO to avoid the large percentage increases in unit fees necessitated by the available 
coins of the realm. The combination of a complex relay-set in the telephone exchange and a coinbox mechanism containing several hundred moving parts is an obvious reliability hazard; perhaps less obvious are the difficulties of managing a system where the responsibility for the major parts is divided between the essentially separate internal and external maintenance staffs. The difficulty of attributing faults to the appropriate part of the system is probably the largest contributory factor causing recurrent call failure.
Full cash-boxes had always been a problem, both in rented payphones and in Public Call Offices. It is by no means trivial: a full cashbox generally causes coins to jam, often resulting in a visit by an engineer. Service can often be restored by shaking the coins down and clearing the jam ; but unless the cash-box is emptied immediately, the trouble will quickly recur. A number of attempts had been made to detect and report coin jams in POA boxes, with only limited success.

Taken from 
Pay-on-Answer Coinboxes - An extract from J R Lewin article in  
  BTEJ April 1982

Payphone Test equipment

Coins 10p A MAX,A MIN and R MIN

Payphone Test Weights
Payphone Go weight 
payphone NOGO

Neil goes to answer the pay phone that has been ringing for the first 3 minutes of The Young Ones. Bambi. Series 2 Episode 1. Just as he get there it stops ringing 

I have little original information on this era of pay phones, but I am interested in them because I remember them and used them a lot.

There is a good article in the Post office electrical engineers' Journal (POEEJ) Vol51 Part 4 Jan 1959 that is dedicated to STD.
it can be found on page 343 called "The pay-on-answer Coin-Box System"

That describes both the box and exchange kit in good detail.

There is also an article later on in the same journal vol 54 part 3 October 1961  on page 176 describing "a pay-on-answer coin-box for subscribers' installations"
The Exchange kit was called "Coin and Fee Checking Equipment", there is a picture of the equipment used to test this kit at

Hywel Clatworthy,
12 Mar 2011, 13:27