New Update for NL920 UK Unlocked - 2/4/2013

I'm on EE and had to change my phone, kept getting people telling me they could not hear me, despite holding the phone next to my mouth, it was a known problem. Your problem sounds very similar.

Was this after or before update? My phone was totally fine till 1308 update! Now I have 2 of my Lumias on 1308 and the other two still on 1249. Both on 1308 magically developed "I can't hear you" issue!
 
It was before, back in January. Quite a few people at the time were having the same issue, there are a few threads on it. I believe in my case it was a hardware issue, so may be different than your problem if yours is software related. Do you get the same problem when using Bluetooth? Mine was fine then, only occurred when using the phone direct.