You said you did everything Fairphone recommended. Did you also reboot your Device in safe mode?
(Push Power Button as if you want to shut down your Phone but then longpress on “Shut Down” on the screen. Then it restarts in safe mode. Note that you have to put your widgets back on the screens if you restart in normal mode. But safe mode doesn’t affect your data.)
In Safe Mode all your downloaded apps are inactiv. If the Problem doesn’t occur in safe mode one of your installed apps may cause the problem. Therefore you can only uninstall one by one and check. (Be sure to backup data or sync with google).
That was one of the things I’ve tried because of my network connection problem. But in my case I had absolutely no connection or a very strong connection. But no on/off as you described it. So maybe it’s another issue if you loose connection during phone calls.