Cellular Continuitywas introduced by Apple in iOS 8 which allows users to answer phone calls through Mac or iPad when their iPhone is away. However, this feature required iPhone, Mac or iPad in the same Wi-Fi Network.
With iOS 9, you don’t need your iPhone to be on the same network as your iPad or Mac to receive a call.
T- Mobile has come the first carrier to support cellular Continuity and it is expected that other carriers will also support this feature once iOS 9 is released to public. Find out how to enable Cellular Connectivity on your iOS 9 and El Capitan powered devices. You will be required to sign in to the same iCloud Account on all your devices.
Enable Cellular Continuity on iPhone, iPad or Mac
1. Launch “Settings” >> Tap “Phone” options.
2. Tap “Calls on Other Devices”.
3. Enable the “Allow Calls on Other Devices” button.
4. Tap “Add Wi-Fi Calling For Other Devices”.
On iPad (Mac)
1. Launch “Settings” and tap “FaceTime”.
2. Tap and enable “Calls from iPhone” option.
3. Tap “Upgrade to Wi-Fi calling” option and tap “Enable”.
4. Enter the code given on your iPhone to activate Wi-Fi calling on iPad.
5. Tap Allow
6. Wait for Wi-Fi calls to be activated on your iPad.