[Serusers] Prepaid Billing Issues

Hi, I looking for a prepaid biiling solution for my SER. I have already read that I have to use B2BUA or Asterisk with some billing app. I have a Cisco 2610 with 2 FXO so the Asterisk would not talk with the PSTN directly. 1- First I ask you what is better and scalable to use? 2- How I loads or add credits to its DB? (I know that B2BUA use a Radius, so I would use FreeRadius+MySQL) 3- How B2BUA or Asterisk control not a user pass his credits? How they calc the credit-time-price_perminute, and make the user hang up a call? 4- Do you know a free billing app, make it in PHP/MySQL for an easy personalization? 5- THANK YOU VERY MUCH!!! Nahuel Ramos.