After a bit more testing, it appears that the first incoming call usually works ok, but on the 2nd incoming call, I lose sound and have to reboot.
I've got the journalctl logs for the first incoming call (1.txt) which worked, and then the 2nd incoming call (2.txt) which didn't.
I can't understand why this only happens to me? I've tried a clean install and it's still the same, and I've even tried on my old Droid 4 (which is falling to bits) and it's the same on that too.
Are incoming calls really working fine for everyone else?

EDIT: I don't really know what I'm doing here, but have tried a few things in case they help...
In /etc/pulse/daemon.conf I've added:
log-target = file:/home/nemo/pa.log
log-level = debug

I then rebooted, and called myself, and on this occasion I lost sound on the first call (which happens sometimes). I've attached the pa.log that it created.
When I try pacmd, it says 'Daemon not responding.', so don't know if that means it's crashed?

EDIT2: Well, I've found that I can restart pulseaudio without rebooting by doing:

pkill -9 pulseaudio
pulseaudio -vvvv --start -n --file=/etc/pulse/arm_droid_default.pa

I've tried it in the foreground (without the --start), and when the problem occurs, it seems that the process doesn't abort, but just hangs?
D: [pulseaudio] module-suspend-on-idle.c: Sink sink.null becomes idle, timeout in 1 seconds.
D: [pulseaudio] policy-group.c: Starting to move sink input feedback-event
D: [pulseaudio] dbusif.c: Policy groups moving: 9
D: [pulseaudio] droid-sink.c: Sink set port to parking
I: [pulseaudio] sink.c: Changed port of sink 1 "sink.primary" to output-parking

Thanks