I seem to remember that you could get to various destinations, including directory enquiries, by rapidly tapping the number on the handset rest in a pulse tone coin box, even without inserting any coins in it! (I was age 6 or 7 at that time.)
There have been a few things you could do with pay phones over the years. My favourite one was where a certain model of BT payphone allowed you to place a call as normal, then as long as you had at least 11p of credit left you could press the "follow on call" button, then dial 99* and your coin would be returned.
Another thing you could do with some models was dial the operator (100) with a DTMF dialler. This would put you through to them without the "cuckoo" tones sounding, so if the operator wasn't being otherwise alert, they would not realise you were calling from a payphone. This meant you could just ask them to connect a call for you.
As a random aside, there were also some randomly scattered 0800 numbers which connected to operator services but for some reason they did not know which number you were calling from. You would have expected they would refuse to do anything but in actual fact you could ask for a call to be connected and they would just ask which number you were calling from and then connect you.
That's enough for now