Outputting ringtone disables incoming audio
With a birch device you can make as many outbound calls as you like. The first inbound call reconfigures pulseaudio in such a way that inbound voice no longer works.
Inspection of the amixer settings shows no difference before, during and after the call.
Attached are the pulseausdio settings immediately after boot and then during the call where inbound voice is disabled.
The interesting difference to note here are that the wm8962 sink sample rate has been reconfigured to 44.1KHz as well as the ources.