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.