It’s easy to think that Skype is the only kid on the block, but its VOIP competitors are hitting significant numbers. Rebtel, the world’s second largest VOIP network, announces today that it has hit 15 million users worldwide and has increased revenues by more than 55% in 2011 to $60 million in revenue. The company attributes this to its transparency, honesty, and new innovations, such as the KeepTalking technology that lets users switch seamlessly between between WIFI and 3G if the smartphone app senses the connection quality is deteriorating. In comparison to Rebtel’s 15 million users, Skype has 663 million registered users as of September 2011.

The nice user numbers and financials come at critical time in the company’s history as Rebtel is currently growing at a rate of 500,000 users per month. Rebtel claims word of mouth recommendations are an integral part of its growth as it drives registrations and accounts for over two-thirds of new user registrations.

The company was founded in Sweden in 2006, has since seen a total of 500 million calls made and 2 billion minutes of international calls logged. The company has been profitable since 2010 and estimates its average revenue per user (ARPU) is three times higher than Skype; Skype currently operates an ARPU of around $8 per month while Rebtel’s ARPU is around $24.

Andreas Bernström, CEO at Rebtel, says, “2012 will be more of the same. We are focused on growing our user base, we aim to add up to ten million to our user base this year, as well as maintaining high revenue run rates and ensuring our apps are viral as they can be.” The company expects to hit $95 million in revenue run rate end of 2012.