Users wanting to call home from abroad are increasingly turning to Skype to the detriment of international carriers, new data reveals.

"Skype is now the largest provider of cross-border communications in the world, by far," claims Stephan Beckert, analyst at research firm TeleGeography.

Skype's technology allows consumers to make cheap long-distance calls over the internet. It is mostly used on desktops but Skype has made the move into mobile too and it now comes pre-installed on some mobile handsets.

According to the firm's data, over the past 25 years, international call volume from telephones has grown at a compounded annual rate of 15%. In the past two years this growth has slowed to only 8%, rising from 376 billion minutes in 2008 to an estimated 406 billion minutes last year.

By comparison, Skype's on-net international traffic between two Skype users grew 51% in 2008, and is projected to grow 63% in 2009, to 54 billion minutes. "The volume of traffic routed via Skype is tremendous," says Beckert. "Demand for international voice has been remarkably robust, but it's clearly not recession-proof."

Established in 2003 and based in Luxembourg, privately owned Skype has more than 520 million registered customers who use the free web service for voice, video or text communication.

But despite its size, its revenue is relatively modest - at about $551 million in 2008 - as the company has had a difficult time getting users to pay for its largely free services.

Skype aims to nearly double its annual revenue to $1 billion in two years.