Changing 2-factor Authorization Without Trusted Number or Device

My father has forgotten his old trusted phone number and does not have the phone. How can he sign in to change/turn off 2 step verification (which requires the phone number)? I've tried iforgot and Apple ID, but each method requires knowing the old phone number (nobody wrote it down). His Apple ID and password are still fine, but I cannot get past this verification text/phone call. He was just trying to add an app to his iPad.