This is probably a long shot, but running the latest nightly, I am having trouble getting Phone Audio to work with my car across Bluetooth. I can't pinpoint exactly which nightly it stopped working, but it was fine around the beginning of April. My car has a Uconnect 8.4AN system and it can successfully pair with my phone, access my phone's phonebook, and stream music from my phone. However, when I try to make a phone call, the phone on the receiving end rings, but I can't hear the other person through my phone or car speakers, and the other person can't hear anything either. It's the same deal the other way around -- I can answer a call, but neither party hears anything.
Everything works fine using a Nexus 5 with a stock ROM, so I'm guessing there's a bug at play here, or I'm missing something obvious.