Q: iPhone 6plus : Why Can't People Hear Me During A Phone Call?

I have the iPhone 6plus 64 gb gold..just few days ago. It seems that when I call or receive phone calls, the other party cannot hear me yet I can hear them perfectly. When I use Facetime and other social apps people on the other end can hear me just fine. I checked my settings and turned the phone off then on again but the problem still exists. Ialready brought it to apple service provider to diagnose the problem. And they say there is no problem in the mic piece. They don't even replace it coz its still working according to their engineer. My iphone was restored and then update to the latest Os but still the problems exist. HELP!