You're best bet would be to get online and see what in what area the area code attached to the pay phone is located, I'd think. Unless you call it back, get really lucky, and someone answers who can tell you the answer. Honestly, though, the whole Angel Investigations name thing aside, investigating isn't the thing we're best at.
Maybe ask Veronica for help? She's one of those actual detective types, rumor has it.
And don't mention it about the car. Just... be careful with it. Please. Yourself, too, of course.