It was four old pennies when I was a kid (I'm not sure whether you could use anything else, like ha'pennies).When you remember subscribers (as they were called then) having to wait months to get a telephone installed. My uncle was a sales rep and was in that position after moving house. He had to go to a phone box with pocketfuls of coppers* every morning to make his calls to the office and to clients.
* Also, when you remember calling small change 'coppers' after the material they were made of.
I can remember when it was a source of pride (and affluence) that someone was the only one in the road who had a landline (not us). Very occasionally there would be a knock at the door - it was the phone owner, there was an urgent call for us, typically a bereavement or similar. The phone owner had allowed the number to be passed on in emergencies. I can remember making such a call from a phone box on Boxing Day after my grandmother passed away so the neighbour could pass the message on to her sister.
After we had a phone, I seem to remember (but this may have been an urban myth) ringing home from a box and pressing button 'B' quickly at the first ring to get the money back, a single ring being a pre-arranged code 'I'm on my way home', 'I'm at the station', 'All is well', whatever.