Unlimited means they just do it, no points, no rolls or anything. I am not sure if that is only supposed to be among themselves (i.e. Altha to Altha), or with any intelligent creature, though.
I imagine that the powers on the Telepathy psion list would work as normal, and allow them to increase their telepathic range, but at the stated power point cost (i.e. if a telepathy power is a rank 10 power and allows them to mind speak at 500 meters, then if they spend 10 PP, & use the power, they can speak at 500 meters instead of 50 feet).