Just to add: I've tried ringing some of them but when answered, people seem very reticent to give me a location. I've only been able to get information from about 1 in 5 calls.
I have about 20 numbers. I'm trying to locate someone who rings from phone boxes: don't worry, it's nothing dodgy, just someone who has gone missing who I would like to find (or just pin down to a particular area)