Users are reporting a good deal of audio breakup on incoming calls, although the person on the other send of the conversation can hear things just fine. Walking toward the device usually helps, and some users saying turning off the iPhone's cellular data clears up the problem. People are even posting videos demonstrating the issue. It doesn't seem to be a matter of range. People are getting the audio 'stuttering' even with 5 bars of signal strength.
I've experienced the problem with my MicroCell as well. AT&T thought the hardware was bad, and cheerfully exchanged it. The problem, although intermittent, remained.