If you have a HTC One M9 and are on EE in the UK, then you will soon be able to make calls over a wifi connection - ideal for if your house is a mobile blackspot.
Unlike say, Skype or O2's TuGo app, wifi calling is baked into the handset's normal phone app so you can make and receive calls and texts as though your phone was connected directly to an ordinary mobile transmitter.
According to Cool Smartphone in order to use the new functionality you'll need to install the latest system update, which will be rolling out shortly. Apparently once enabled, your phone will monitor both wifi strength and cell transmitter strength to figure out the best way to route your call.
The new feature isn't unique the M9 - the technology is slowly being rolled out across different devices and networks. EE already supports the iPhone 6 and Samsung Galaxy S6, for example. [Cool Smartphone]