Two factor authentication for a phone number that's no longer available
Hello:
My son locked his Iphone after forgetting his passcode. After several attempts we decided to factory reset it. THE problem: the phone had its number changed long time ago.. and that new number was never updated on his Apple ID account. So we know go to an Imac to his Icloud account and when trying to recover his info (before proceeding in the phone's setup) it asks for the code which is sent to a number no longer available. So if I click on "Didnt get a verification code?" I go to "More Options.." Is asking me to confirm the phone #... and then next page: "Can't get verification code" I select the 'continue' on the "Can't use your phone number?" option. That takes me to the "Update your phone number from one of our devices" page.
Followed instructions on my Imac and added his new phone # to my trusted devices on my Imac by using the call method. After proceeding, I do not receive the code that the phone is 'sending' to my devices. We cant find a way to verify that he is the owner of his Icloud account. What step am I missing? Please help. Thanks in advance.
iPhone 12, iOS 16