Happened yesterday with my HTC Surround. I could hear them fine, but they heard nothing. Tried a couple more test calls with same result. Turned it off and back on, seems to be ok now, but now I'm nervous abt. it.
It seems like the two metal wires undernear the hood on bottom left (where the Windows logo is) sometimes have problem spring up to touch the connector plate on the cover. This happens could be because when I change battery I might have pressed the back to hard and pushed the metal wires down (caused deformation). My fix is to put small rubber to hold the meta wires up so there is always a connection.
That fixed it and I don't have issue anymore. Other than that I can't complain about the phone and all the features since the price is much more affordable for me.
I had that problem too. Probably the phone was still in headphones mode even though I pulled out the earplugs. Inserting the earplugs into the slot and pulling it out solved the problem for me. Maybe that's why a simple reset solves it too??