I own a full blowing billing solution. This was writen initalial for 2 german mobil phone operators. Any as the solution works on CDR (Call detail record) i can be modified to bill your customer base. The system is able to handle and bill millions of subscribers.
If you need more information. I willing to lower my bid if you can proide a specification how you pricing struction will be and how many subscribers you need to handle.
Looking forward to hear from you...