How can I unlink my phone number from an old Apple Account I’ve forgotten?

Hello.Recently, I created a new account on my iPhone 16 Pro. During registration, I provided my email and phone number, received the codes, and set up the phone. However, my phone number is not displayed in the settings, and when I try to add it, a message appears stating that this phone number is already being used in another account.

Many years ago, I already had an iPhone and an account associated with my number. Apple probably still recognizes it. But when I try to find the account using my phone number, I get a message saying "Apple ID is invalid or not supported."

When I try to reset the password and enter my current email (I don't remember which one I used many years ago), the password for my current account gets reset.

Could you please advise what I can do?

[Re-Titled by Moderator]

Original Title: Could not add the mobile number