The same happens to me, only with a Dutch phone number and S10.
I may be wrong, but I think this started happening after I accepted some "software update" that popped up a few months ago over my home screen. I have to say that there was an out of usual feeling to this pop-up. It looked like a regular update that even required restarting the phone.
Some time after that screen showed up (per your screenshot above), I entered my number and then... nothing really different (the popup didn't show up anymore) until I noticed my phone bills where higher than usual (like +€5 per week).
My provider mentions about this kind of paid service (some FTMSC service or so) in the following page (sorry it's in Dutch):
There, they suggest going to https://www.payinfo.nl/ and unsubscribing from the paid service.
That worked, but then the popup started to show up again.
So now I am wondering about that update.
Does this ring a bell to anyone?
In general I would appreciate a word from Samsung Support Services, or any advise like "how to get a list of updates applied" as to find out if there is any rogue update that was applied to my device.
This is really annoying! How do I fix it? Thank you