@robpike I know that "own phone number" was an afterthought in the whole system. At least on non-iphones it is very common for a phone to not know the number of the inserted SIM card, because it's not stored/not stored correctly. (Usually this is no problem since the phone doesn't need it to work.)
It might be related to that?

Some operators have a short code that you can send/text in order for them to actively push the correct configuration (again) to fix this.