Yes, I've tried a factory reset. Didn't even use Smart Switch, only the necessary backup to my Samsung Account, moved the files (docs, pictures, videos, etc.) back by copy-pasting it from my laptop, even installed all my apps manually.
Didn't help even a bit.
I think it has to be something in my settings or another app that is causing the interference.
I do have another idea I can try out, which is gonna take even more time then the full reset...
Revoke the permission to the microphone of every app that uses it, except for of course the Samsung phone app and the call recording app, or even uninstall them.
Since I have to make a call or be called after every change I described above.
But this is gonna take forever I'm afraid...
So, any other suggestions for finding out what causes this issue would be very welcome.
May I ask in which country do you live?
And do you have any other recorder app of any kind installed on your phone?
I did have this same issue with my Galaxy S9, back then I've tried ACR call recorder from NLL, Call Recorder Cube ACR, the one from Boldbeast Software Inc. and the one I'm using now, even paid for the full version of 2 of these. Since the update to Android 9 I've had this problem.