I was a little worried about activating based on all of the horror stories, but I was able to add lines through our business account for my and my wife's new LTE Apple Watches. Here were the steps. This is a crazy long process, but trust me it works. Read carefully and don't miss the details. First, this is a business plan. Will differ for personal plans Have the box the watch came in handy - you will need numbers off of it Call 866-288-7629 Get someone on the line to add new service Tell them you're activating an Apple Watch Give them the IMEI and long number starting with EID when asked Let them go through their activation steps Once they say they're done, keep them on the line Turn watch on/off When back on go to the watch app on the phone and look under cellular. You're looking for it to say "AT&T not active" or something to that effect Once you see that, go on a computer and go to this address: https://m.att.com/my/#/welcome?orig...ughAction.myworld?actionType=ManageNumberSync Enter the number of the phone you are pairing with and only the phone number and IMEI, not the zip. This is the PHONE's IMEI and phone number, not the watch's This is setting up a new login for numbersync, so dont think this has anything to do with any existing AT&T logins you may have Once it's all set up go back to the watch app on the phone under cellular and hit "set up cellular" or something to that effect It will prompt you for the numbersync username and password you set up above That should do it. Good luck!