I had an issue that was similar, but not the same. When I was at home on wifi and answered calls, I could hear the caller but they couldn't hear me (said there was weird squeaking noise or something). Answering calls when not on wifi gave me no issues, but my apartment is a total dead-zone so I really need wifi calling.
I just fixed it today actually by doing two things: turning OFF power saving mode, and turning off the voice control that deals with answering calls and such. I'm not sure if just one or both of those steps were necessary, but wow, F power saving mode! I thought my phone was pretty fast before, but w/ power saving mode off it just absolutely blazes. I've had pretty awesome battery life on 4.3 Tweaked ROM w/ Saber kernel, so I don't think I'll mind a battery hit in exchange for the ridiculous performance I'm seeing.
Back to your problem, I've heard of people solving issues by cleaning the microphone with a toothpick or something. Are you able to use the mic for voice recording or voice commands or anything? If you're having a hardware problem with the mic, an exchange might be necessary.