I am having literally the EXACT same problem! After a week of trying to get ahold of somebody who knows what they are talking about, I'm on the phone with AT&T right now waiting for my Microcell to restart so that I can make a test call.
I will report back with what happens.
EDIT: They told me that it is a known issue. They had me make some calls so that they could have logs, but they said it might take a while for it to be fixed =/ They have until Monday, that's when the return period ends for me.
My latest call has been going for 56 minutes now with no issues. It seems fairly sporadic. I also forgot to mention that I am connected to work via VPN on the same network the Microcell is running on. I'm sure that I am pushing some bandwidth constraints today, but will keep you posted. I will be doing the same tomorrow. I'll have over 600 minutes logged on the microcell since I got it. I have been getting input from folks on the conference call and the feedback I have been getting is that when it is working fine, it does not sounds like I am on a cell phone. I'm coming in louder than the presenters on the call.
-Benster