I’m an avid apple-avoider so this is logic based, not specific experience based, but many systems have more than one recovery option – is there an email address connected to the account?
Alternatively, as you obviously still have access to the device, is there a way to edit or add to the recovery options, or will Apple insist on getting confirmation from the existing phone number?
"Man is born in freedom, but he soon becomes a slave, in cages of convention, from the cradle, to the grave."