I got this from an AT&T executive and confirmed by an Apple exec 3 days ago.
IOS 12 contained an update that closed/shut down the Facetime API that was used by AT&T to facilitate NumberSync.
As a result, AT&T NumberSync can no longer use Facetime to make voice calls over cellular.
In response to the Facetime API closure by Apple, AT&T must develop its own phone app to be installed on IOS devices so those devices can resume making and receiving NumberSync voice calls over cellular.
This app was unfortunately not released before Apple closed the Facetime API.
Between now and when AT&T releases the NumberSync Phone App for iPad, there is no permanent solution to this problem unless Apple opens up the Facetime API.
The App is expected to be released soon.
Obviously, the exact release date cannot be guaranteed due to the nature of software development but the goal of AT&T Is to hopefully get it released within 30 days.
Once it is released, I will need to download it to my iPad and set it up in accordance with AT&T NumberSync set-up info that will be provided when the app is released.
The AT&T exec will give me a call when it is released to make sure I am able to get it set up properly, so I can resume using this critically important NumberSync feature on my iPad.
The following work around “might work” but is not guaranteed due to the API closure.
Workaround:
Reset network setting on iPhone and then it will prompted you to update carrier settings.
Then enable Wi-fi call on device
1. On your iPhone, go to Settings > Phone and turn on Wi-Fi Calling, then turn on Calls on Other Devices.
2. Tap Add Wi-Fi Calling For Other Devices.
3. On your other devices, sign in to iCloud and FaceTime with the same Apple ID and password that you use on your iPhone.
4. Turn on Wi-Fi Calling:
o On your iPad or iPod touch: Go to Settings > FaceTime > Calls from iPhone and tap Upgrade to Wi-Fi Calling.
o On your Mac: Open FaceTime and choose FaceTime > Preferences > Settings and select Calls From iPhone > Upgrade to Wi-Fi Calling.
5. When a six-digit code appears, enter it on your iPhone and tap Allow.