I wish I had a good answer to that. I'v noticed myself that it definately is not consist. Im American and sometimes I could get a call on my cell, When I lived in the Uk I could never get one (I was on a pay as you go contract then) and now I live in S. Korea and I can always get a call to my cell phone. Maybe - to be less technical- it depends on whether to cell phone and pay phone are both on compatible networks or from competing ones.
In S. Korea a call can be made from or to a cell anywhere in the country regardless of networks for the same charges/fees. Where as in the states theres is all the network competition and regional roaming charges.

A very basic and non-technical answer, I know - but plausible.