Same here (Nexus 5) it seems is a pulseaudio issue, can't switch profile to headset_head_unit when trying to use bluetooth headset/car kit microphone
profiles:
a2dp_sink: High Fidelity Playback (A2DP Sink) (priority 10, available: unknown)
headset_head_unit: Headset Head Unit (HSP/HFP) (priority 20, available: unknown)
off: Off (priority 0, available: yes)
active profile: off
I manage to force it to switch to A2DP but of course no mic
profiles:
a2dp_sink: High Fidelity Playback (A2DP Sink) (priority 10, available: unknown)
headset_head_unit: Headset Head Unit (HSP/HFP) (priority 20, available: unknown)
off: Off (priority 0, available: yes)
active profile: <a2dp_sink>