Wi-Fi Calling seems to be the new "in" tech-thing of the moment.

With customers driving the demand for always on connectivity, mobile phone networks are turning to Wi-Fi to enable services that their own masts might not be able to provide.

That's a good thing for those who want to remain connected in an area that perhaps doesn't support a particular network, letting you take calls and get messages where the cellular network might not reach.

But what is Wi-Fi Calling, what does the service offer, who supports it and on what devices?

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Wi-Fi Calling is exactly what you're thinking: a feature that allows you to make and receive calls (and send text messages) over a Wi-Fi network instead of a traditional mobile network.

More specifically, with Wi-Fi Calling, cellular packets of a phone call or text message are transferred over the internet from your mobile device to your carrier (whilst utilising higher data speeds than most cellular connections), then piped through the cellular network, and finally delivered to the intended recipient. Simples, right?

It's a handy feature should you find yourself in an area with poor signal, because it lets you take advantage of Wi-Fi so your device is connected. That might be in rural areas with no network masts by using domestic Wi-Fi, or a basement nightclub that has Wi-Fi.

To use Wi-Fi Calling you need two things. You need a network that supports it (which we list below) and you need a device that supports it (also listed below).

However, there's a slight catch in that you probably need a device sold by the network you intend to use it on, so you have the right software on it to access the network service. 

In the case of EE - the first network in the UK to launch Wi-Fi Calling - you will have to have bought your phone directly from them to use Wi-Fi Calling. If you have an unlocked handset you've moved from another network, or you bought a grey import, there's a good chance you won't be able to use the service.

For many customers on a regular monthly payment plan, with a handset from their network, there should be no problem at all, as long as that handset supports Wi-Fi Calling.

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Several popular apps - including messenger apps like WhatsApp and Skype - allow you to send and receive calls/texts over Wi-Fi or your data network, but this isn't Wi-Fi Calling.

When using an app like WhatsApp, your calling activity is kept strictly within the app itself and between users of that app. Your phone number is used to verify your identity, but it has nothing to do with the network at all, other than the data you use.

Similarly, Apple's FaceTime and iMessage hand off calling and messages to internet services using the device's data connection, but that's an Apple service, rather than from your network.

In some cases, like iMessage, you get unlimited messaging and it's seamlessly integrated for use between Apple device users, but it's distinctly different from the proposition around network Wi-Fi Calling, where messages you send and calls you make will (probably) be deducted from your talk plan.

If you're an O2 or Three customer, this might all sound familiar. O2 has a service called TU Go and Three has a service called InTouch.

These will allow you to make calls over Wi-Fi, but you have to be using the apps for it to work. It's a separate solution and the beauty of Wi-Fi Calling is that it's seamless and integrated, without the need for an additional app on your phone.

The idea is that it's a lot simpler, because with network-enabled Wi-Fi Calling, you literally don't have to do anything at all, it just works.

The major downside to Wi-Fi Calling is that ongoing phone calls cannot always transfer back to your mobile network when kicked off Wi-Fi, yet.

So, if you start a call on Wi-Fi while at Starbucks, but then wander out of range, your mobile network won't be able to pick up the call. That is until the networks provision VoLTE (voice over LTE). 

Currently, when you place a voice call, your device switches to 2G or 3G and you cannot move a call to or from Wi-Fi. However, if or when your network supports VoLTE (also called 4G Voice) then you'll be able to move that voice call from Wi-Fi back to the network without dropping your call.

In the UK, EE has confirmed that it is working on this transfer and 4G Voice is expected in late 2015. Vodafone has also confirmed it's testing voice over 4G and we suspect it will arrive later in 2015 too, but that's unconfirmed.

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Here's a list of networks that currently enable Wi-Fi Calling in the UK and US:

  • EE enabled Wi-Fi Calling on its network in April 2015
  • Vodafone switched on its Wi-Fi Calling service in September 2015

O2 and Three haven't enabled Wi-Fi Calling yet, but they currently offer apps (called TU Go and InTouch) with similar functionality, as mentioned above.

  • T-Mobile enabled Wi-Fi Calling when Apple introduced the feature with iOS 8
  • Sprint enabled Wi-Fi Calling on its network after Apple expanded support with the iOS 8.3 update

AT&T and Verizon have also confirmed plans to enable Wi-Fi Calling in 2015. Keep in mind AT&T CEO's, Ralph De la Vega, has said his company won't enable Wi-Fi Calling until it can also offer VoLTE for a seamless handoff.

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Here's a list of devices that currently support Wi-Fi Calling, with links to their websites for more information about the devices as well as how to turn on the feature:

  • EE UK (website) - iPhone 5S, iPhone 5C, iPhone 6, iPhone 6 Plus, Samsung Galaxy S5, Samsung Galaxy S6, Samsung Galaxy S6 Edge, Microsoft Lumia 640, HTC One M9, EE Harrier
  • T-Mobile USA (website) promised to enable Wi-Fi Calling for all its devices
  • Sprint USA (website) - iPhone 5C, iPhone 5S, iPhone 6, iPhone 6 Plus

Naturally, this list will change over time, so you can expect wider support than just those devices we've mentioned. EE has confirmed that iPhone 6S and iPhone 6S Plus and the Sony Xperia Z5 and Xperia Z5 Compact will offer Wi-Fi calling too.

Although Vodafone has enabled the service in the UK, it is yet to confirm the devices that support Wi-Fi Calling.

One of the important things to note about Wi-Fi Calling is that it needs carrier/network software to enable it. In the case of EE in the UK, you have to have bought your device from EE for it to work with Wi-Fi Caling. If you buy your device elsewhere, it lacks the network-specific software it needs for the service to function, as we talk about above.

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Your phone should come with an instruction manual that explains how to set up Wi-Fi Calling, but you could just go to your carrier's website (links above) to find step-by-step guides. Most of the time you'll need to switch on an option under Settings, and perhaps notify your carrier that you plan to use Wi-Fi Calling.

Once you're set up, simply use your phone as normal. You should see a new symbol (see top image) at the top of your phone when using Wi-Fi Calling. Keep in mind the other person/recipient doesn't need a device that supports Wi-Fi Calling (nor does he or she need to enable it with a carrier) for the feature to work.

Pocket-lint's Wi-Fi Calling hub has all the latest and breaking news on the feature.