I wouldn't take the phone that's immature. Its not yours to take and your only dating your not married. Although he should be being honest with you that he has a friend that is a girl or telling you why this girl is calling, if there is nothing to hide. I would ask him flat out, "I see that so/so has called alot, who is she to you". If he gets all fidgety and excusatory then he's hiding something. If he comes out and tells you her name is so/so, do you want to meet her, then do so. Dont know how long you have been seeing him but he should be dealing with your relationship and your feelings respectfully and honestly at all times. Beware though if he is not willing to share the fact that he has a friend that is a girl, which there is nothing wrong with, then he could lie about it too. You have to use your intuition on this one. Ask him to meet her if he doesnt volunteer it up and if he says no, then dump him. Whether it is man or woman that his is just "Friends" with as your partner he should want to share them with you not hide them. Think, look and listen. Good luck.
Okay well this has happened to me before. I think you should just talk to him and see what he has to say. Listen to him and go with what he says. Tell him you would really appreciate it if he would stop talking to her so much. Hope this helped