The same problem is coming back more often now, and around every 3 calls give me sound only. I did not use the headphone since the last reboot, but the speakers only. It seems to me that there is a fundamental problem about switching to different audio out devices : small speaker vs loudspeaker vs headphone.
To fairphone people, which I’m sure read this forum every now and then, this thread shows that many people are impacted, and it has been reported almost a year ago.
Could it be time to take action ? Maybe first give feedback that the problem has been taken into account.
Many here have already reported the problem to the reseller, which only suggested doing a factory reset. This has been tested and does not help