Apparently the operator didn't have a way of knowing the number you were asking to make a call to was a phone booth,

They did.

In the UK, when a payphone _answered_ a call, the payphone played a "cuckoo" tone (beep-boop) for about ten seconds. This alerted the operator not to allow reverse charge calls. Of course in more recent yeas an operator probably hasn't a clue what the payphone cuckoo means, hence you getting away with it.