Adding phone number to Apple ID account
Error when editing Apple ID account setting phone number “already in use on another account”
See -> If you've forgotten your Apple ID email address or phone number
If you've forgotten your Apple ID email address or phone number – Apple Support (UK)
Sign in to your Apple ID account page and edit / update your Apple ID details.
-> edit your trusted phone number.
Do you have another Apple ID that already linked to that phone number?
Get more help -> Contact Apple Support -> Get Support (apple.com)
