I've had this happen with a few phones.
Do you sometimes use a wired headset? From what I've gathered in my situations were the phone seemed to think the headset was still connected, even after I removed it. Plugging the headset back in and then unplugging it fixed the problem.
My phone has been doing that since the update. If i restart my phone it fixes the issue. It only does it sometimes so I'm not not sure what the issue is.
I have the exact same problem since the kit kat update. Probably 20% of the time when someone calls me they can't hear what I say and hang up. Call back same problem. Reset the phone and the phone works properly. Never had a problem with the phone until the kit kat update.
Same thing happens to me as well. I have to restart the phone to get it working again. Very frustrating and have it happened on a few business calls. Very unprofessional.