If you've had an issue enabling this feature on your iOS devices, you're not alone. I, personally, found that my second iPad Air 2 was not able to make these phone calls over LTE when my iPhone was off or not near me. I had followed AT&T's directions to a 'T' and still, I was not able to make it happen. All the settings on my iPhone 6S Plus and iPad Air 2 were exactly as they requested and yet it was not working.
If you are like me, you've probably thought, "Well, that's okay. Apple's 'continuity' Feature is enough for me. After all, that allows calling on the iPad when the iPhone is nearby and both devices are on wi-fi, which for all intents and purposes is going to cover most use-case scenarios."
Well, fortunately you don't have to settle for that. Number Sync allows you to make calls on iPads and Apple Watch as well as Mac OS X devices (when all devices are running the correct version of the OS AND when they are on the compatible list ) when the phone is off (or away from you) and whether or not the iPad is on Wi-Fi. Note: for the Watch to work, you have to be in a 'known' wi-if network zone, i.e. you have to have used the watch and phone together in that same place in the past while the phone was on that wi-if network at that time, otherwise the watch will not have that network in its 'know network' list in its software. Before you ask, no, there is no way that you can look at that list on the watch or edit it.
So...the one detail that was missing in AT&T's instructions was that when you turn on wi-if calling on the secondary device (tablet), IT MUST BE ON WI-FI when provisioning the wi-if calling. To me, this was not necessarily common sense (, to you, it may be). I had my device on LTE through the whole process, thinking that it was the LTE Network that was provisioning this Number Sync feature and had nothing to do with my personal wi-if home network. Alas, I was wrong. I had turned all the settings on, but still the process was stuck on this last 'provisioning step' on the iPad. I turned the features all off and started over again on the iPhone and iPad. Finally, it occurred to me to flip on the wi-if on my tablet. The moment I did that, everything clicked into place and the provisioning was complete. I then tried the feature on all devices, turning my iPhone 6S Plus off and making a regular phone call with my iPad Air 2 on LTE. It worked seamlessly. I also tried it on my Apple Watch since I was at home and my Network would be a known Network to the Watch. Also, that worked seamlessly as well.
Yay. Success.
I hope this little detail might help some of you that may have been stymied by this feature to find resolution like it did me.