On Friday, 23 June 2017 00:49:11 UTC+1, Steve Walker wrote:
I think it was only in the '80s that they changed the tone received by
an operator from a payphone - until then they could not tell them from
private phones and people had learned that you could make free reverse
charges calls *to* phoneboxes.

This was because the 1980s GPO/BT owned all the payphones and knew which lines were coinbox lines. It was possible to make reverse charge calls to a payphone because the operator could ask the person at the payphone to insert money to pay for the call (and on the old Button A/B boxes listen to the separate 'dings' for sixpences and shillings).

When private payphones were introduced they were connected to ordinary lines so the operators had to have some means of knowing a payphone was connected, hence the 'cuckoo' tone.

Owain