For assistance with access to your phone number, check with your carrier. But I’m guessing the phone number wasn’t hacked, and the phone wasn’t hacked, but rather the Apple Account was compromised and a two-factor authentication phone number was added, or a two-factor phone number was changed. That sort of compromise is less common when two-factor authentication is in use and when passwords are not re-used, but some attackers will try to phish the verification code, or will try to get both the password and the code.
You can try Recovery: https://support.apple.com/en-us/118574
Recovery might allow access to the Apple Account with a lost number. Recovery won’t allow access to a thoroughly compromised Apple Account, though. Those accounts effectively have new owners.