[Galaxy S 4 Mini Original Android Development] _cosi_:
Just tested with nightly 29 and it still breaks. I pair with a UConnect hands free car kit, built in Fiat/Chrysler cars.

I have also tried to re-pair the phone, disallowing access to phone contacts and messages. The issue persists.

The outgoing call is established:

01-29 16:29:12.526 8751 8751 I Telecom : CallAudioManager: connecting bluetooth true
01-29 16:29:12.543 8751 8751 I Telecom : NewOutgoingCallIntentBroadcaster: Sending NewOutgoingCallBroadcast for [113509573, CONNECTING, null, tel:**************, A, childs(0), has_parent(false), [[Capabilities:]], [[Properties:]], false, ComponentInfo{com.android.phone/com.android.services.telephony.TelephonyConnectionService}, [356a192b7913b04c54574d18c28d46e6395428ab], UserHandle{0}]

When the first ringtone should be heard, it seems that bluetooth audio is disconnected:

01-29 16:29:14.569 8751 9552 I Telecom : CallsManager: setCallState CONNECTING -> DIALING, call: [113509573, CONNECTING, com.android.phone/com.android.services.telephony.TelephonyConnectionService, tel:**************, A, childs(0), has_parent(false), [[Capabilities: CAPABILITY_SUPPORT_HOLD CAPABILITY_MUTE]], [[Properties:]], false, ComponentInfo{com.android.phone/com.android.services.telephony.TelephonyConnectionService}, [356a192b7913b04c54574d18c28d46e6395428ab], UserHandle{0}]
...
01-29 16:29:14.579 8751 9552 I Telecom : BluetoothPhoneService: updateHeadsetWithCallState numActive 0, numHeld 0, callState 3, ringing number [da39a3ee5e6b4b0d3255bfef95601890afd80709], ringing type 128
01-29 16:29:14.585 8538 8871 I bt_a2dp_hw: adev_set_parameters: state 3
01-29 16:29:14.585 8538 8871 I bt_a2dp_hw: out_set_parameters: out_set_parameters: state 3
01-29 16:29:14.585 8538 8871 I bt_a2dp_hw: a2dp_command: A2DP COMMAND A2DP_CTRL_CMD_CHECK_STREAM_STARTED
01-29 16:29:14.585 8751 9552 I Telecom : InCallController: Sending updateCall [113509573, DIALING, com.android.phone/com.android.services.telephony.TelephonyConnectionService, tel:**************, A, childs(0), has_parent(false), [[Capabilities: CAPABILITY_SUPPORT_HOLD CAPABILITY_MUTE]], [[Properties:]], false, ComponentInfo{com.android.phone/com.android.services.telephony.TelephonyConnectionService}, [356a192b7913b04c54574d18c28d46e6395428ab], UserHandle{0}] ==> [InCall@1, parent:null, children:[]]
...
01-29 16:29:21.075 8751 9672 I Telecom : InCallController: Sending updateCall [113509573, DIALING, com.android.phone/com.android.services.telephony.TelephonyConnectionService, tel:**************, A, childs(0), has_parent(false), [[Capabilities: CAPABILITY_SUPPORT_HOLD CAPABILITY_MUTE]], [[Properties:]], false, ComponentInfo{com.android.phone/com.android.services.telephony.TelephonyConnectionService}, [356a192b7913b04c54574d18c28d46e6395428ab], UserHandle{0}] ==> [InCall@1, parent:null, children:[]]
01-29 16:29:21.075 8751 9672 I Telecom : InCallController: Components updated: [ComponentInfo{com.android.dialer/com.android.incallui.InCallServiceImpl}]
01-29 16:29:22.675 9875 9936 W bt_l2cap: L2CAP - st: CLOSED evt: 10
01-29 16:29:22.678 9875 9936 W bt_l2cap: L2CAP - st: TERM_W4_SEC_COMP evt: 7

The call is still continued on the local phone, car HMI shows that phone is available but not the active call. I disconnect the ongoing dialing call from the phone:

01-29 16:29:31.357 8751 8767 I Telecom : Event: Call 2: REQUEST_DISCONNECT, null

Lateron the system does a watchdog reset

01-29 16:30:55.114 8751 9325 E Watchdog: Triggering SysRq for system_server watchdog
01-29 16:30:55.127 8751 9325 W Watchdog: *** WATCHDOG KILLING SYSTEM PROCESS: Blocked in handler on main thread (main)
01-29 16:30:55.127 8751 9325 W Watchdog: main thread stack trace:
01-29 16:30:55.127 8751 9325 W Watchdog: at android.media.AudioSystem.isMicrophoneMuted(Native Method)
01-29 16:30:55.127 8751 9325 W Watchdog: at android.media.AudioManager.isMicrophoneMute(AudioManager.java:1597)
01-29 16:30:55.127 8751 9325 W Watchdog: at com.android.server.telecom.CallAudioManager$1.handleMessage(CallAudioManager.java:99)
01-29 16:30:55.127 8751 9325 W Watchdog: at android.os.Handler.dispatchMessage(Handler.java:102)
01-29 16:30:55.127 8751 9325 W Watchdog: at android.os.Looper.loop(Looper.java:148)
01-29 16:30:55.127 8751 9325 W Watchdog: at com.android.server.SystemServer.run(SystemServer.java:307)
01-29 16:30:55.127 8751 9325 W Watchdog: at com.android.server.SystemServer.main(SystemServer.java:179)
01-29 16:30:55.127 8751 9325 W Watchdog: at java.lang.reflect.Method.invoke(Native Method)
01-29 16:30:55.127 8751 9325 W Watchdog: at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
01-29 16:30:55.127 8751 9325 W Watchdog: at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
01-29 16:30:55.127 8751 9325 W Watchdog: *** GOODBYE!

I can send you the full log as PM if this helps.