The app subscription and purchased phone number - both are associated with your Apple ID.
- Please make sure that you had and still have an active subscription which is not interrupted in recent times.
- Also, be sure that you have not changed Apple ID - i.e. subscribed through a different Apple ID than the one trying to restore through.
If these are still true and you are unable to restore your number, please go through the guidelines to get the best support, raise the issue so we can help you out.