What to Do if My New Phone Number Is Already Associated with an Apple Account?

If you're trying to use this number for iMessage, you should deregister it from Apple's system. You can do this by going to their deregistration webpage and entering your number under the section that says 'No longer have your iPhone?' Just keep in mind that since the number is associated with another account, the other steps won't work for you.

I had a similar issue a while back, and I found out that it helps to contact customer support for your phone service provider. They might have the tools to clear up the issue on their end.

Have you ever used a different Apple account with this number before? Sometimes past accounts can linger and cause confusion like this.