Since moving to the 48 network, my phone keeps asking for my phone number several times a day, usually just after I unlock the screen:

It is a Samsung A51 5G that I originally purchased from Three. It never did this when I was with Three. The phone works fine with phone calls, SMS and data, so I don't know why it keeps asking for my number.

When I enter my phone number, the pop-up disappears until later in the day the next time I unlock the screen. I also tried dropping the '0' and touching 'Skip', but this screen still pops up later when I unlock the screen.  I tried asking on Samsung's forum, but got no answer other than to try my number again.