Originally posted by ev607797:
Some of the independent telcos here had post pay phones {.....} There were also pay phones that did not return dial tone until coins were deposited.
Here's the way it worked in England (for anyone interested enough in the history!).
The older pre-pay boxes gave dialtone as soon as you picked up the handset, but contacts on the coin mechanism kept the dial shorted out until the correct amount for a local call had been deposited. The first coin going into the mechanism also applied a short across the transmitter, so after depositing the correct amount the caller could dial the number and hear his party answer, but could not talk.
As I mentioned above, coin control was under the sole control of the caller. Upon hearing an answer he had to press button "A" on the front of the phone which deposited the coins into the box and restored all the coin contacts to normal so that he could speak. If there was no answer, the wrong number, busy, etc. he pressed button "B" on the side of the box which dumped the coins into the return chute. The dial and transmitter contacts were restored to normal, but this button also operated a small spring-wound timer contact which opened the line for a few seconds to clear down any connection.
An extra set of contacts was fitted to the dial which opened when it was rotated right round to zero and didn't close again until it returned to rest. These were wired in series with the coin mechanism contacts which shorted out the dial, and thus allowed the caller to dial zero for operator without depositing any coins. After preparing the call, the operator would, of course, ask for the correct amount.
The contacts which shorted out the handset transmitter were changeover types and actually switched between the handset and a small carbon transmitter fitted inside the box. The first coin going into the slot therefore muted the main transmitter and turned on the coin transmitter so that the operator could hear the bell/gong signals from both that and any subsequent coins. From then on the call went as before, with the caller having to press button "A" if successful or button "B" to get his money back. There was no way to speak to the operator again without releasing the coin contacts, so any repeat attempts had to be made by pressing "B" to clear the call and then starting over.
The fact that the handset transmitter was "live" until the first coin was deposited also meant that the phone would accept incoming calls easily. The special dial contacts were adjusted to allow a 9 to be dialed as well as a zero after the 999 emergency service came into operation.
The post-pay phones which came in after STD arrived in the late 1950s/1960s were quite different, as they had been designed to allow fully automatic, direct-dialed long-distance calls without operator intervention. The coin slots had a restricting lock which was released by a solenoid, so you could not deposit coins at all until the C.O. equipment allowed it. Once a coin was pushed (and I do mean
pushed and not dropped -- you had to press fairly hard) into the mechanism, it went through the detectors and straight into the box with no way of returning it (the return chute was purely to catch and reject damaged or otherwise unacceptable coins).
You could pick up and dial any call, local or long-distance (once available in the area) with no coin deposit. When answer supervision was returned from the distant end, the C.O. would open the slots and apply "pay tone" to the line (heard by both parties) to signal that a deposit was now required. The signal returned from the phone to the C.O. then allowed the call to proceed based upon whatever time interval was allowed for the particular call. When the time expired, the coinbox relay sets then repeated the process to ask for more money.
If coins were not deposited within a few seconds the connection was dropped and the slots locked again.
The pre-pay A/B phones were mostly displaced as STD spread throughout the country, although there was at least one still in use (in a remote part of Scotland) in the 1980s. The post-pay boxes were pretty much the standard through the 1970s. They too were phased out during the 1980s as the modern microprocessor controlled units started to take over.
Edited to add: You can see the old pre- and post-payment phones here (about a third of the way down the page):
https://www.telephonesuk.co.uk/kiosks_payphones.htm The A/B boxes are obvious; the 705/735 are the post-pay phones that I described above. There are internal views of a couple.
I maintained/vended Protel COCOTs (with the Elcotel Series 5 boards) a few years ago, and other than a placing a regular test call and a coin verification test, most of my other testing was accomplished by using telemetry codes.