Phone app not working on iPhone 5S

We purchased an iphone 5S 32gb Grey/Black for my mom yesterday and today the phone app does not seem to be working properly. When she makes outgoing calls, there will be a loud constant beep and she cannot hear the other person nor can the party on the other line hear her. That was earlier this afternoon. Now, the phone app not only has that loud beeping sound, but sometimes you already cannot be heard at all using either speaker or regular phone. I was thinking the problem was with the mic or the speakers but then facetime works, Siri works (can hear me and be heard) and the problem comes and goes which leads me to believe the problem might be software related. Anyone out there having the same problems? I would appreciate any feedback.