Have you tried restarting everything and see if you can add the number to the desired account? Sometimes it can take up to 24 hours for changes to sync.
If necessary, there is a contact link.
Apple ID Support
Apple ID Support (2)
Apple Support by e-mail or chat Work through the screens to get a chat option. You may need to pick a problem that is slightly different than yours to get a chat option.
Apple Support using Twitter
Or fill out and submit this form. The form is for iTunes, but it will get passed to the ID Support section.