Depends on D20022. Ref T13222. Since you can easily lock yourself out of your account by swapping to a bad number, prevent contact number edits while "contact number" MFA (today, always SMS) is enabled.
(Another approach would be to bind factors to specific contact numbers, and then prevent that number from being edited or disabled while SMS MFA was attached to it. However, I think that's a bit more complicated and a little more unwieldy, and ends up in about the same place as this. I'd consider it more strongly in the future if we had like 20 users say "I have 9 phones" but I doubt this is a real use case.)