Hello RNR86,
Welcome to the community!
Sorry to hear you're having issues changing your number. You should note that the forums are community-driven and not intended as a venue for customer services. We are only able to assist with general information issues. Unfortunately, your issue appears to be related to your account or a technical issue.
If you wish to change your number, you may need to contact customer service. I understand there may be a charge if they change the number for you (see here). However, they will be able to access your account and possibly determine why you're receiving the error you note. In addition, they can also be contacted via Live Chat, Facebook, or Twitter. Those methods can be accessed via the contact page posted above.
Hope this helps 😀
