Update phone number without access to old phone number

Trying to help another TurboTax user update their phone number on their profile. She does not have access to the old phone number yet the Intuit/TurboTax account only lets you change the phone number by sending a code to (or calling) the old number.  If somebody doesn’t have access to the old number, it’s not possible to change the number on their account… even the Customer Service personnel we are on the phone with can’t figure this out… already on the phone with them for a half-hour, still no resolution and they don’t know what else to do.