frustration

How can I get good reception on the iphone 6?

I bought an iphone 6 a little over a month ago. People constantly tell me they can't hear me. I did factory reset and network reset (Verizon), I then went to genius bar at which time they replaced the phone saying I had a bad one. Well, it's a week later and I'm still having the same problem! Verizon basically said there's nothing that they can do because it seems to be a problem with iphone 6, not with them. There's nothing covering the upper mic. I hold my phone away from the mic and my hair isn't interfering. I've never had these issues with any other cell phone. Extremely frustrating!