sounds like he's got someone else on the side.
So ask him. Don't accuse him of anything. Simply ask him who the girl was who called.
If he tells you a friend and that nothing is happening...say ok and thank him for being honest. Tell him that he doesn't need to keep secrets from you. Tell him that it's ok with you if he has female friends. Show him that you are a mature woman. Tell him you want to meet this girl and you three should have lunch.
Then when his jaw hits the floor and he stammers about not being able to or has some other reason for not meeting with her...then you'll know he's cheating.
My x used to do that then look me in the eyes and say I love you. Your the only girl I could ever want or look at turns out he had another girl on the side he puts his fone on silent supposey him so no one will bother him while he is with the one person who Really matters. Bullshyt he puts it on silent just incase the other girl calls or texts him. Yu need to Really figure out waht is going on. Us gurls, we are not stupid. We get that gut feeling when yu know he is don't doing something wrong now ask yourself are you getting that gut feeling? And if not, is he sincere or do yu just not care?
He's either got some else (sorry) or he doesn't wanna be distracted while with you. So either Mad good or mad bad...
Must admit it sounds as if he has someone else aswell as you and is to afraid to tell you or dosent know which one of you to choose. If it was all inncent he wouldn′t have acted as nothing happend and the girl would have said who she was on the phone. Terribly sorry. :(
1) He probably doesn't want any phonecalls interrupting his time with you.
2) It was VERY VERY wrong of you to answer his phone with him ask you to or giving you permission to do so.
3) You can either take his word or decide that he's cheating and forget about him.
Next time you two are alone in an intimate setting and he puts his phone on silent, say to him "If you honestly do love me, hand me your phone and let me view the call history and saved numbers."
If he refuses or puts the blame on you for some perverted reason, he has something to hide and is not totally honest with you. You be the judge whether you want to live with any dis-trust.