[CriminalMind]

Well, well well... this might have been the problem. Not sure, but there was a fix for it posted Wednesday:

Merge "Fix occasional completely mute incoming call" into ics

241 final PhoneApp app = PhoneApp.getInstance();
242
243 // If the ringer is currently ringing and/or vibrating, stop it
244 - // right now (before actually answering the call.)
245 - app.getRinger().stopRing();
244 + // right now and prevent new rings (before actually answering the call.)
245 + app.notifier.silenceRinger();
246
247 boolean answered = false;
248 Phone phone = ringing.getPhone();