My iPhone has the previous owner's number listed as "My Number", how do I remove it??

A friend gave me her old iPhone 11 when she upgraded. I fully wiped the phone, and set it up on my own AppleID to be used as a camera. It has a lot of storage so it works great as a youtube filming device. The phone has no SIM and is not activated on any cellular network.

So it all works great and I put out tons of youtube videos with excellent video quality.

THE PROBLEM is.... every once in a while I do something that doesn't work, like try to facetime from this 11, to my own primary iPhone. If I could see what the other one's camera sees, it would help me line up my shot without needing a cameraman.

Problem is I can't facetime with this old phone. And when I poke around settings, I believe the reason why is that the old phone still lists my friend's phone number as "My Number". Everywhere I've seen it, it is a read only block of text. Her number is not attached to my AppleID, mine is. I can't make facetime calls as her on my regular phone or accept them - or texts. But her old phone is confused and still thinks that it has her number.

How can I remove her number from this phone. I just want it to be a simple "iPod Touch" essentially. Wifi only! I'm looking all over and I can't figure it out.