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(Pocket-lint) - Mobile operator O2 is the latest UK network to offer 4G and Wi-Fi calling for its customers. The new service replaces the TU Go app, and means O2 customers with a compatible phone - for now just an iPhone 7 or iPhone 7 Plus - can make and receive calls over Wi-Fi when there's little to no regular signal.

If however you do have good 4G signal, then O2 says the 4G calling function will provide clearer call quality and improved indoor coverage. If you are on a call and transfer from a 4G area to a Wi-Fi area, or vice versa, your phone will simply transfer to whichever signal is strongest without affecting your call.

You don't need to worry about 4G calling using your data, either, as the service uses your minutes instead. Wi-Fi and 4G calling is only available to O2 customers on pay monthly, SIM-only or business tariffs and these should receive a notification for a software update very soon. O2 says more devices will be added over time as well.

The service is also currently limited to London, Leeds and Slough, but more UK cities will be added over the coming months. 

Before enabling Wi-Fi and 4G calling, you'll need to make sure your iPhone 7 or iPhone 7 Plus is running iOS 10.3 or above. To update your phone, follow these instructions:

  1. Go to Settings > General > Software Update
  2. Download and install any software update that pops up, if none, move on to the next step
  3. If you do install a new software update, you may get a notification to update your carrier settings, if you do, select "Update"

Once you're running the latest software, you can then activate Wi-Fi calling by following these steps:

  1. Go to Settings > Phone
  2. Select "Wi-Fi Calling"
  3. Toggle "Wi-Fi Calling on this iPhone" switch to the on position
  4. Confirm that you want to activate the feature

To activate 4G calling, follow these steps:

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  1. Go to Settings > Mobile Data
  2. Select "Enable 4G"
  3. Select "Voice and Data"
Writing by Max Langridge.