While we wait for Ting to release an official carrier profile, to get WiFi calling working you will need to create a custom "Carrier Bundle" for the Ting SIM card.
This guide is not for the faint of heart. I've only tested this on my unlocked iPhone 6 plus - it may not work on other devices, although it probably will. You need a computer running iTunes.
- Install the CommCenter patch from this repo in Cydia: apt.chinasnow.net
- Install iFile on your device and navigate to /var/mobile/Library/Carrier Bundles/iPhone
- Create a ZIP file of TMobile_US.bundle using iFile and email it to your PC. If the file is missing, go to Settings > General > About and click "Update" when prompted to download a carrier update.
- On your PC, extract the ZIP and open carrier.plist in a plist editor (icopybot.com has one for free). Look for <key>SupportedSIMs</key>. After that, there will be several numbers listed. Add <string>310260</string> anywhere in the list. This is the network ID that the SIM card identifies as and will enable the T-Mobile profile to be used with this SIM card.
- Create a new folder called "Payload" and place TMobile_US.bundle into this folder.
- Create a ZIP file with "Payload" in its root. Double check that this was successful - when you open the ZIP file, you should see the "Payload" folder.
- Rename the ZIP file to ting.ipcc
- Run this command to allow custom carrier updates: "C:\Program Files\iTunes\iTunes.exe" /setPrefInt carrier-testing 1
- Connect your phone to iTunes. Shift-click the "Update" button. Under the file-type selector, pick "iPhone/iPad Carrier Configuration files" and select "ting.ipcc".
- Go to Settings > General > About on your phone. If you were successful, "T-Mobile" should appear under the Carrier field. If you see "Carrier 18.0" or similar, the iPhone is not detecting a compatible carrier bundle for your SIM card. You may have a different network ID than I do.
- Go to Settings > Phone > Wi-Fi Calls to enable the feature. Make sure the feature is enabled in your Ting Account Settings.
- After a few seconds, you should see "T-Mobile Wi-Fi" in the carrier name.
To remove this hacked profile, go to /var/mobile/Library/Carrier Bundles. Delete the iPhone folder and the contents of the Overlay folder. iOS will re-create the iPhone folder when the phone reboots.