I love the 8100, but I have had (3) of them go dead:
The microphone stops working, for in and outgoing calls.
It happened on the first one abruptly, after dropping it in my car on the carpet. The second one I did not drop, and it's outgoing audio became disabled on incoming calls only, then in/out calls. The third one went dead in 5minutes.
So far, all have been exchanged, and I have purchased the "warranty" from T-mobile, so no issues with that yet.

Is it possible I am doing something to disable the mic? Seems unlikely to me.

Are you somehow pushing the mute button on top of the phone?

A drop can permanently disable the microphone for both inbound and outbound calls. We've seen this on a previous 7100. The bluetooth headset was the only way to use the phone after a drop. Luckily, we haven't dropped the 8100.

As phones get smaller (with corresponding less absorption for a drop), it becomes necessary to have a case/skin that offers some padding OR just never drop it.

The drop could be a possible reason on the first one, but it was not a factor on the second and third.