But now Skype may be 'threatened' by O2, which has launched an app which lets users make and receive phone calls and texts via a tablet, computer or smartphone.
The app called Tu Go is available for Android, Apple's iOS devices and Windows 7 PCs but limited to "pay monthly" subscribers - so excludes corporate accounts.
O2's owner Telefonica has experimented with Voip (voice over internet protocol) before with its Tu Me app which was launched in 2012 with limited success.
There are already dozens of Voip apps on the market including lesser-known names such as Tango, Fring, Bria and Zerofone as well as manufacturer's own services including BlackBerry BBM and Apple's Facetime.
But the Ovum telecoms consultancy believes Tu Go will stand out from the crowd.
And already O2 is responding to competition from Microsoft's Skype unit and other Voip services - but would it be able to replace the more famous and efficient Skype? Time will tell.
Read more about it at: BBC News