Phone calls made from mobiles traditionally use the network of masts you see dotted around the countryside and throughout cities.
However, as you'll have discovered the reception can often be patchy, especially in rural areas or deep inside buildings.
That’s where Wi–Fi calling comes in.
So what is Wi-Fi calling?
Quite simply, Wi–Fi calling allows you to make phone calls and send text messages over wireless internet networks rather than over the 3G or 4G mobile internet connection that you normally use and which is generated by mobile phone masts.
All sorts of Wi-Fi networks are compatible with the service. That includes the one you pay your broadband provider for at your home, as well as a public one in a coffee shop, airport or train station.
So how do I get started?
The crucial thing to know is that different networks offer different Wi–Fi calling options.
This can be confusing, so if you live in a place where mobile reception is bad and you want to make calls over Wi–Fi, it pays to do your research.
Which networks offer Wi-Fi calling?
UK carriers that offer Wi-Fi calling in some form are EE, Vodafone, Three and O2.
Wi-Fi calling on Vodafone and EE
EE and Vodafone offer Wi–Fi calling schemes that are essentially hassle-free and don't require you to do much to get started.
And they make it just as easy to make calls.
If you are somewhere without mobile reception but with Wi–Fi, such as an underground station, you only need to connect your smartphone to the network to make a call. It's that simple.
But you’ll need to set it up first
Wi–Fi calling is easy on EE and Vodafone, but you need to follow specific instructions to ensure that your smartphone allows it to work in the first place.
EE offers a comprehensive guide on its website, with easy to follow steps that even the biggest technophobes can easily get their heads around.
Likewise, Vodafone has a dedicated page in order to make things a breeze.
You need to stay in the same Wi–Fi area
If you make a call over Wi–Fi and then move out of range of that network, the call will fail.
As yet, networks do not let you switch from a Wi–Fi call to a normal mobile call seamlessly.
Vodafone says it’s working hard on new technology to make this happen, with plans to launch later in 2016.
You’ll need the right phone
Not all smartphones can make calls over Wi–Fi.
Vodafone only lets owners of the iPhone 6s, iPhone 6s Plus, Samsung Galaxy S6 and Samsung Galaxy S6 Edge Plus make Wi-Fi calls.
And even then you must buy the latter two devices directly from it.
EE has a wider range of handsets that work with Wi–Fi calling, with a string of handsets from the likes of Apple, Samsung and LG compatible with its service.
You'll need the right contract too
Unfortunately, Wi–Fi calling is not available for every mobile customer.
EE offers the service on two pay as you go phones, its Harrier Mini and Microsoft’s Lumia 550, as well as 87 phones on its pay monthly plans.
Vodafone only allows pay monthly customers to use its Wi–Fi calling.
Wi-Fi calling on Three and O2
The Wi-Fi calling schemes operated by Three and O2 operate on fundementally the same principles. But there a few differences, as we'll see below.
Three and O2 Wi-Fi calls require an app
Rather than simply switching on Wi–Fi calling in your smartphone’s settings, you need to use a special app to make calls.
O2 has an app called TU Go, while Three has one called InTouch.
Both are available from Apple’s App Store and Google Play and are easy to use.
However, they add an extra layer of complexity to proceedings.
Popular services such as Skype, Viber and WhatsApp also offer the chance to make calls over Wi–Fi.
However, like Three and O2’s tools, these require you to download an app and create a profile.
If you want to keep things extra simple, it’s best to go with what your preferred network has to offer.