Billing is indeed an issue to us, we looked into quite a few. It seems that most do the job of initial simple voice set up for prepaid and postpaid, yet at the end of the day, when investigating operators with 1000 ports and more, as well as vendors like Cisco, Alcatel, Nortel, Vocaltec, Veraz, Netcentrex and many others they all claim that for alternative cariers high-end solutions like Portal and MIND CTI actually do the job at the end of the day..