So the watch is not activated despite what you may be seeing online. There really is not point in un-pairing and re-pairing the watch. You can under cellular see if it will let you remove the plan and then try to re-add it. But its not the watch, it something on ATT's back-end, sounds like NumberSync is not fully processing. Is the line the watch on the original one ATT gave it or has it changed?
It added the line directly from the watch during cellular setup. I think I might call att and see if they can push number sync through? What do you think?