@BobS Sounds dissapointing. Some errors are well know to me and were all related to the proximity sensor, the think that should turn your screen off when our making a call, not working reliably. I used to do all kind of stuff with my ear touched the display while it was not turned of, amongst others, activating the Flight Mode (awww - i hate that!), changing the time zone or setting other language. This is probably due to thew quick settings getting opened and buttons pressed by my ear. Especially the Flight mode is very prominently placed their.
I can say it got better with time, i seem to subconsciously holding it right now though i still think the proximity sensor is the worst part of the current FP. I experienced it works better with shorter hair tough the idea of getting a haircut for a phone to work is pretty strange :-). What helped me was turning the screen off manually by shortly pressing the power button as soon i got a connection when making calls. Annoyingly but effective. Or try to cover the sensor (small circle directly left of the earspeaker at all times.
Good luck!