Phone call sounds garbled and robotic

I haven't encountered it for a about a week; it had been a few days since it last happened when I posted the original question. As for the questions posted above, I haven't been able to figure out a pattern. The few times it had happened I was driving and my phone was connected to my car via bluetooth. I was in different locations, only separated by a few miles, and with high coverage. In two instances it occurred while connecting to a mobile phone not on the T-mobile network and another time it occurred while connecting to a land-line.

I've been unable to reproduce the problem consistently, and since it has only happened while in my car, I usually just stop trying to make the call until I can restart the phone. I can try airplane mode next time it occurs. Come to think of it, may it be possible that something in my car is messing with my signal? I've noticed the GPS has issues finding my location and updating while doing direction, but I just figured it was the device itself.