hello, i got same problem on nexus 4 running 4.4.2 android.
i risked and rooted phone just to have possibility to view system logs when this occurs, and it looks like the same errors happens in this kind situation, but the problem is that google can't help much with them
04-09 11:27:27.177 E/ (177): csd_client_disable_device: Disable received in invalid state:1
04-09 11:27:27.177 E/ACDB-LOADER(177): Error: ACDB AudProc vol returned = -8
04-09 11:27:27.177 E/ (177): csd_client_enable_device: Enable received in invalid state 1
and such error is in logs too often when trying to make phone call:
4-09 11:27:27.487 E/BluetoothPhoneService(863): Could not get a handle on Connection object for the call
Any advice would be great