Why not? Really, IMHO, it is pretty harmless, for the most part, for him to spend hours talking... For a guy, it is a pretty valuable thing for talking to be something he is good at.... I don't mean that to be a slam on guys, but I remember 14 year old guys, and the ones that could to more that grunt at a girl were nice to have around!
And, the time he spends at home talking on the phone is time he is spending at home and not elsewhere. He will want to talk to these girls whether you encourage it or not.... so - I would embrace it - get him his own phone (without long distance of course) and at least you know where he is!