Mobile VoIP provider Rebtel today released some impressive numbers, announcing that it is growing at a rate of 500,000 users per month.

Rebtel, which launched in 2006, says it has surpassed 15 million total users, with over 500 million calls and two billion minutes of international calls logged.

The company has been profitable since 2010, boasting average revenue per user (ARPU) that it says is three times higher than Skype.

Skype, which was purchased by Microsoft for $8.5 billion last year, currently operates an ARPU of around $8 per month, while Rebtel's ARPU is around $24. Rebtel's figures show it increased revenues by more than 55 percent in 2011 and hit $60 million in revenue. The company predicts revenue run rate for December 2012 lies at $95 million.

By comparison, in its first quarter earnings call, Microsoft reported 170 million Skype subscribers, which spend an average of 125 minutes per month on the service.

"2011 was a great year for us," said Andreas Bernström, CEO at Rebtel, in a statement. "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."