Three is the latest mobile operator to jump on the Wi-Fi calling bandwagon after launching its service in the UK. The company joins a list of operators across the world in letting customers stay connected by piggybacking on Wi-Fi networks, allowing them to send and receive calls and messages even in cellular-stricken areas.

Wi-Fi calling is nothing new. Services like Skype have been offering voice calls over Wi-Fi connection for years, and more recently apps like Facebook, Viber and WhatsApp have allowed users to call each other using something called Voice over Internet Protocol, or VoIP, which uses an internet connection instead of a phone line.

So what's the different between these services and Three's "embedded" service? Hasn't it been offering Wi-Fi calling for years anyway? Well, yes and no. Allow us to explain further with a crash course in calling tech.

What is Wi-Fi calling?

Wi-Fi calling is exactly what it sounds like: calling, texting or otherwise messaging over a WLAN network rather than the mobile network you pay your operator for. Wi-Fi calling as it's sold by like of Three, EE and Vodafone, is slightly different to those offered by third-party services, though. Rather than needing an app that allows Wi-Fi calling, their technology is 'native', which means the functionality is integrated into the network itself.

There are a number of benefits to this. For starters, users can call and message their contacts from their phone's built-in dialler or contacts app, meaning you don't have to load up a separate application every time you want to make a Wi-Fi call. It also means you don't have to manually activate the feature whenever your mobile signal runs low – Wi-Fi calling is always running in the background, and the moment your 2G, 3G or 4G signal disappears, you phone will automatically hand over telephony functions to an available Wi-Fi network.

Which networks support Wi-Fi calling?

All four of the major UK mobile operators offer Wi-Fi calling in some capacity, but only EE, Vodafone and Three offer it as a fully-embedded service. Three's InTouch app has now been replaced by integrated Wi-Fi calling, leaving O2 as the last major mobile network in the UK to only offer a separate Wi-Fi calling service, which it provides through its TU Go smartphone app.

Does my phone support Wi-Fi calling?

To get Wi-Fi calling on your phone, it needs to be compatible with the software from your mobile operator that enables the functionality on the network. This is usually limited to newer phones when the service is initially released and then expanded to older handsets over time.

Three's Wi-Fi calling service, for example, is currently only available on iPhones from the 5C onwards, the LG G5 and Samsung Galaxy S6/S6 Edge. The Galaxy S7 range will be added next month, as we presume will other handsets.

For Android users with compatible smartphones, Wi-Fi calling should be activated automatically. iPhone users need to do just a little bit more need to have iOS 10.2 or above installed; once it is, they should go to 'Settings>Phone>Wi-Fi Calling' and switch the feature on manually.