Can't change phone # on IPhone 7

3/7/2021 - My wife got an IPhone 12 and I inherited her 7. It won't let me change "My Number" in Settings from hers to mine. Sometimes I do get to another screen where it says "primary" and there's one more screen after that that shows her number, but none let me change that number. I tried every suggestion I saw - turning off IMessage and Facetime and then the phone - no good. I backed up to ICloud and reset the phone - no good. This shouldn't be difficult, but for me it is. Any more suggestions? Thanks.