iPhone unavailable

iPhone 13 pro.

i switched from 3 to Sky recently.

The day after the switch I got a message on my phone saying iPhone Unavailable try again in 3 minutes.

since then almost every day I get a similar message, wait 3, 9 or 15 minutes.

I understand that this message is what you get if you type your passcode incorrectly too many times, but I haven't typed the code in at all, I'm using Face Id.

I can be using the phone one minute, return it to my shirt pocket and the next time I take it out there's the message. I have turned the phone off and on again without effect.

Any ideas?