[Serusers] Prepaid Billing Issues

[Nahuel Alejandro Ramos]

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.