I ended up having to exchange my phone today because the mic stopped working for phone calls. I could hear people on the other end, but they couldn't hear me unless I plugged in headphones or put on speaker phone. Coincidentally Bing search would still work.
I tried turning the phone off and on, plugging and unplugging the headphones, cleaning the headphone jack, and resetting the phone. Unfortunately none of these options worked.
Just figured I'd throw this out there in case anyone else runs into this issue or has a fix for it.
This probably is NOT the same problem you had, but I discovered that it's very easy to accidently cover the mic hole that's on the bottom edge of the phone with your finger. And as soon as you cover the hole, the other party can't hear a thing. This happened to me once without realizing the cause, so I hung up and tried again -- twice -- before I realized the problem. I've still done it once since then, but realized it as soon as I realized the person couldn't hear me anymore. Kinda like the infamous iPhone antennae problem... but different. :-)
Its an issue with the headphone jack, I've had the exact same problem with my Lumia 920. I've returned 3 phones so far for this exact same problem. On this recent replacement phone I got I'm scared of plugging in earphones cuz, I think it'll mess up again. I havent yet pinpoint the reason why the headphone jack acts as if the earphones are still plugged in, but I suspect its got something to do with the earphones inline music control.
Yeah, I ended up returning that phone... the new one (knock on wood) hasn't had the problem yet, though it has given me a few scares where the mic stops working, but will come back. Generally it occurs now when I have the phone charging and the headphone jack plugged into the car. I'm thinking one of the firmware updates might be forcing it to reset the mic since so far it hasn't been permanently broken.