new iphone 5s keeps dropping calls and voice cuts out

I am new to cell phones...never thought I needed one...got one as a gift....loved it . Iphone 5c....worked great. I lost it ( no insurance etc. I feel so stupid)....anyway, I got a new 5s......drops calls constantly ..my voice cuts out so the other person is saying "what ..i can't hear you.

ALL THE TIME ! I took it back to Verizen...they tried a couple things...didn't help...gave me a new one. The same problem ....dropped calls and voice cutting out. This is REALLY starting to **** me off.