I spent about 3 hours dealing with this last night, because I was trying to setup 2 watches with NumberSync (my number with my watch, and husband number with his watch). Here's what I ended up having to do. Go to AT&T account on computer, go to Account Profile, Manage Access ID, Manage Authorized Users, then create and Access ID for the number you are trying to sync. Then when you go back to the watch and follow all of the steps, you will login to AT&T with the new Access ID instead of the one you had been using. I may be forgetting one or two steps somewhere, so feel free to ask questions if I'm not making enough sense.
It all worked perfect after doing that.