Voice over IP (aka VoIP, IP-, internet- or Broadband telephony) is a technology that enables the user to make voice phone calls through the internet using only a data line. Skype (presently owned by Microsoft) is the most prominent software. But Google Hangouts, Viber, Facebook, Yahoo, Sip clients and lot of other apps or programs compete on this market too. Here is a survey of the major software available. In 2015 the widely used social messenger Whatsapp has introduced its own VoIP functionality.
Generally, you need to have a decent and stable 3G or 4G data connection for VoIP. Not having this, you can still use a WI-FI or a landline ADSL provider too. The voice quality is often worse than regular voice calls and you face a longer delay in communication. For those who can live with it, VoIP calls have the lowest international (and sometimes even domestic) call rates from your mobile, tablet or laptop in most countries. To do so, you have to additionally sign up with a VoIP provider before, which you can use wherever you have a decent (and unblocked) data line available. If the line is good enough and you have a cam, you can add video telephony too on many offers.
As a matter of fact, the mobile providers don’t like VoIP very much and have often tried to block it in the past, as they see their earnings twindle. Still, a few countries mostly in the Middle East and some providers do actively block the ports to make VoIP calls, some others ban it officially in their T&Cs only with no further consequences. Where it is known, it’s mentioned in the article. A blocked VoIP access can be unblocked by using a VPN app/software, but this often slows down speed so much, that the line gets too slow for transmitting even a voice call.
The most famous VoIP software charges the highest prices. You can cut prices by using SIP clients and unknown VoIP providers. This is a brilliant and frequently updated guide: http://progx.ch/home-voip-prixbetamax-3-1-1.html. After checking their rates, there is no doubt that VoIP offers the lowest call rates internationally (and sometimes even domestically) at the moment. For more info, check their sites.