On Macbook: Cannot delete a phone number from my Apple ID account

I am attempting to eliminate a phone number from my Apple ID account. When I go to System Preferences> Apple ID> Name, phone, email> Reachable At: there are two phone numbers listed. I want to eliminate one of them. I can click on it and highlight it, but nothing happens when I hit the minus sign. I should note that this phone number is NOT listed in my trusted phone numbers.

The phone number belongs to my wife's iPhone. We share this Apple ID and this is all part of a larger project to have her sign off of my Apple ID and then set up family sharing and other apps like calendar, contacts, notes, etc. At this point, that fact that we cannot eliminate her phone number appears to be complicating our ability to disassociate her MacBook air from Apple ID and iMessage.