At first I had issues but, I now have NumberSync working with my Gear S2 and so far everything is working as intended. Its pretty obvious ATT dropped the ball when it comes to preparing their support organizations. My local ATT store had no clue about NumberSync other than, yea its something new. Telephone support was fairly clueless as well. The person helping me was very polite but, you could tell she was reading the tech support play book.
To activate NumberSync you need to make sure you have the latest update installed. Once installed a NumberSync message will pop up on your device. Just follow the prompts. If you ever need to get back into NumberSync just check the connection settings. If you have the correct update on your phone you should find a menu item for NumberSync.
My first issue was that I could not pair my Gear S2 with my primary line. It would default to my business line even when selecting my personal line during setup. In order to fix the issue, I had to associate my login ID with my personal line. Somehow during one of my may upgrades my business line became associated with my login ID.
The next issue I noticed was battery drain. In my case it turns out that was related to WIFI being on. As soon as I disabled WIFI my battery drain issues seem to have disappeared. I'm not sure what could be causing it, for the moment its working correctly.
I had to re-connect my S2 to NumberSync a second time after my initial pairing. I suspect this was due to what the ATT support person was doing on her end.
Hope this is helpful!