[Serusers] Prepaid approach

Hi, While trying to implement prepaid solution using SER, I decided to go be a intuitive approach which is as follows. -When the user registers, he is placed in a "voip" groups table of ser (or radius server database), i.e he can make only pc-to-pc calls. - Later , or otherwise, when the user registers, he is shifted to a "pstn" groups table of ser , i.e noew he can make only pc-to-pstn calls. -Henceforth, whenever the user logins and tries to make pstn call, the call is approved only if group_radius_is_user_in(username) succeds, else the call rejected by SER. -To restrict the user from making pstn calls when his credits are zero (or beyond a threshold) , a dedicated cron job is scripted to move users from "pstn" group to "voip" when their account_cerdit=0. Am I right in using this approach, or are there any foreseeable problems which I might be overlooking. Thanks. Regards, Ashutosh Kumar