I am having the same issue. I have fixed multiple times, I have used every fix suggested online. Sometimes it temporarily works, but the glitch comes back every time within hours to days. It came back yesterday and I have not been able to get rid of it using steps that have previously worked temporarily, and new steps suggested online. I don’t even have the ability to change my number to remove the 1 in front of the area code, or do anything with a secondary number because I don’t have a secondary number set up. I do not have those settings available in my iPhone XS likely because I don’t have a secondary number set up. This has been going on for months and it is truly disappointing. I was an android user before I bought this iPhone XS. I honestly feel like I should have went with the new Pixel phone. I did not realize how glitchy the software was going to be. I’m very disappointed Apple. What an expensive piece of glass and stainless steel. 😔
But just to confirm if you do get one of the fixes to work, the “Number changed to Primary” glitch will still come back.