Catch 22 when attempting to change a phone number.

When my mother in law set up her Apple ID, she had used a land line phone number, not a mobile. Now when she wants to subscribe and pay for a service, she is told to enter the 2FA code sent to said phone number. The same happens when she tries to change the phone number within the account settings in iTunes. Every time, Apple wants to send the 2FA code to the land line number. This is of course not possible and to top it off, she has moved interstate and doesn't even have that land line number any more.

Is there ANY way to sort this out? I've been looking at things, changed the phone number via a browser but iTunes still shows the old number and all codes are supposedly sent to it.

Any ideas?

Thanks heaps