I couldn't find anything pertaining to this particular problem, but I'm not going to lie and say I went beyond the first few pages and a short search...
Anyway, my phone has been doing this for awhile now, and I'm getting pretty fed up with it..
It happens both when I've dialed out, and when I'm receiving a call. I can talk for anywhere from a few seconds to around two minutes and then the person on the other end can't hear me at all. I've tried seeing if it's because the screen comes back up, even when I don't take the phone away from my face and I'm hitting the mute button, but even though that HAS happened, it doesn't happen every time.
I've also tried taking it away from my face and putting it on speaker phone to see if they can hear me, but that doesn't work all the time either. I can always hear the other person just fine, and when they can hear me, the quality is fine, its just sometimes it sounds to them like I've hung the phone up.
Also, the screen will blank out and come back over and over again sometimes, even when I'm holding the phone to my face, and my ear is covering the sensor.