I'm having the same Issue with an series 6. Trying to activate on an iPhone 7. I had to call ATT to add a line before the setup process let me move forward. Now its been stuck on activating since yesterday. I've tried unpairing and pairing several times, carrier settings are up to date and so is IOS and WatchOS. I have no clue what to do. ATT says everything is OK on their end. The only thing that is out of ordinary is the Phone it's an iPhone 7 because my XS died before christmas so I put the Phone SIM on an old 7 until I change phones.