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For local calls from a public phone, only pennies were needed. They were of course 'old' pre-decimal pennies.
Once the handset was lifted from its cradle, the coins were fed into a holding slot at the top of the box. Then the caller dialled the number that he or she wanted. If someone answered, the caller had to press Button A in order to be heard. If no-one answered, the caller pressed Button B and the coins were returned through a shoot underneath.
In 1951 the cost of a local call up to 5 miles was three old pennies (3d). These calls were not times on A/B boxes. Then price increased to four pennies (4d) on the first of January 1957 and later that year the distance for 4d was increased to 15 miles.
Ian Jolly, one-time telephone engineer
In the late 1960s and early 1970s 4d (1.67p) was the minimum charge for a local call.
I believe the cost went to 2p when we were decimalised in 1971. However the 2p coin was the same size and weight as the old halfpenny and a lot of phones as well as slot machines would accept both coins - which made calls and purchases cheaper!
Larger denomination coins were needed for non-local calls which were known as 'trunk calls'. They normally had to be put by dialing O to be put through to an operator.
With the agreement of the person receiving the call, charges could be reversed by going through the operator. These calls were free to the caller, but I don't know whether the recipient had to pay some sort of premium.
On one occasion my friend and I got exceptional value for our money, albeit with a waste of our time: We made a person to person call to a friend in a London Hall of Residence. It took over an hour to find him and bring him to the phone, but in the end the call cost just a few pence because it was only charged from when he came to the phone and confirmed his name.
At an extra cost, a caller could go via the operator to put a call through to a particular named person. This was useful for expensive trunk calls if the person concerned was likely not to be available. Then the call would cost nothing.
Phone calls were expensive in real terms, and if chatting it was all too easy to run out of coins. Pips would sound and a few seconds later, the line would go dead unless more money was fed in. If the caller expected a long conversation, he or she could feed in a lot of coins at the outset as pressing Button B at the end of the call returned unused coins. However, the process was fiddly and people didn't like to carry a lot of coins around because pre-decimal coins were so large and heavy.