iPhone 6 iOS 9.1 Ends Calls Unexpectedly

iPhone 6

iOS 9.1

Approx. 2 days ago my iPhone 6 started to end calls unexpectedly.

I could be on the phone for a few minutes and my iPhone just ends the call. This happens every 2-5 minutes during a telephone call.

I went to the Verizon Wireless store where they replaced my sim card and sent me on my way. A minute after I walked out of the store, I received a call and within 2 minutes, the call unexpectedly disconnected again. I went right back into the Verizon store, told the individual who had changed on the sim, and she stated it must be the phone. She said I must back up everything on my phone, which she suggested I do at an Apple Store, then went on to tell me I will not get a free replacement of the iPhone, I will have to pay for a new iPhone altogether.

Can anyone suggest a way this issue can be resolved without replacing the phone?