Need to change recovery to a NON-AT&T number on a legacy Yahoo! email account

I need to change the phone number that verifies my Yahoo mail/ameritech.net email account to a non-AT&T number - a number NOT associated with AT&T. I've had this account for 25 years. I have disconnected the phone number that's associated with my Yahoo mail/ameritech.net account. I have followed the instructions and tried to put in the number I want to use for verification (it's a number serviced by T-Mobile), I'm told that AT&T doesn't serve that number.

How do I change the phone number on my Yahoo mail account to a NON-AT&T number?