Apple ID Two Step Authentication

My father just tried to login to his Apple ID - his email address and password work, but the two-step authentication is sending to a landline that is no longer in service.

(It's been a while since he needed to do this.)

How can we reset his login? Recovering the Apple ID still requires confirmation of the old phone number and even when I enter it, it gives us the following error:

"This action could not be completed.

Your request could not be completed because of an error. Try again later."

We're stuck. Any thoughts on next steps?

Thanks for your help.