And keeps growing at a rate of 500,000 newcomers each month.
Rebtel, the self-described 'world's number two mobile VoIP firm behind Skype' has crossed the 20 million user mark.
Customers are able to access the services across landline, iOS, Android, Windows Phone and PC, and it's the wide cross-platform reach that has likely driven app downloads and paying users to grow by more than 250 per cent year-on-year.
The provider claims to be rising at a rate of 500,000 new users a month, and says that average revenue per user is three times than rival Skype's as company revs have doubled from $40m in 2010 to $80m in 2012.
The milestone comes following Rebtel's launch of an SDK that will allow Android and iOS developers to integrate VoIP into their apps in just 15 minutes, while Juniper expects one billion mobile VoIP users by 2017.
Andreas Bernström, CEO at Rebtel, said: "2012 has been another great year, and the outlook for 2013 looks excellent. Our entire team is energised by the potential of our new Voice platform, and the opportunities for further third-party app enhancements it offers.
"We have spent six years improving our service and it’s clearly appreciated by our users with 96 per cent of them saying they would recommend Rebtel to a friend. The ability to give developers the benefit of this value to their users with a branded voice app, without the large infrastructure commitment is very exciting."