Error with number for two factor authentication

We got my grandmother a new phone and cannot sign into her icloud on the new phone as the number connected to the account is incorrect. We do not receive the codes and cannot recover the account as it asks us for the phone number, which ends in 72, and we do not know the number. This number does not belong to any of us, nor has it ever belonged to us, therefore it must be an error on the system as you can't even set up the two factor verification without initially receiving a code on that number. Thus, this is an apple error. It is not an old number of my grandmother or any other family member, and is therefore an error on Apple's side as we could not physically have set up the authentication without access to that number. How can we recover the account without this number (we have the email address and password) I have phoned support and they have been as useless as a sunroof in a submarine.