What is the appropiate age for a girl to talk on the phone with a boy?

The answer depends on the girl, the boy, and the phone. If a boy has a question about homework or a sports team, there is no reason why he can't get the answer by calling a girl. When it comes to calling a girl just to talk, so long as both the boy and girl want to talk it should be appropriate, but their parents should pay attention that the conversations aren't inappropriate.