Rebtel, the second-largest voice-over IP calling company behind Skype, announced major milestones today that prove consumers just can’t get enough of cheap phone calls.

The company says that it has surpassed 15 million connected users (it’s unclear how many of those are active) since its launch in 2006. Rebtel users have made over 500 million calls on the service and have chalked up over two billion minutes of international calls.

Rebtel offers mobile and desktop PC apps that let its users call each other for free, or make inexpensive calls to international numbers. The company’s service is also accessible through any landline or mobile phone without an app. Rebtel just recently released version 2.0 of its iPhone app, which added a first-ever feature to let you hop between voice and data networks during a call.

Not only is Rebtel growing quickly, the company boasts that its average revenue per user (ARPU) is triple Skype’s. Skype now sees a monthly ARPU of $8 per month, while Rebtel’s ARPU is $24. The company increased its revenue by more than 55 percent last year to reach $60 million, and it expects to earn $95 million in 2012.

The Swedish company has spent considerable funds and effort setting up the infrastructure for its service. CEO Andreas Bernstrom told us previously that it brought in $20 million worth of financing from Benchmark Capital and Index Ventures in 2006 to build a system that could connect with operators around the world and terminate voice calls in any country. During 2011, the company put together the data side of its infrastructure, including codecs and stacks to carry voice over data connections.

VoIP call photo via Shutterstock