VOIP billing software

Hi All, ** I'm only looking for individual developer so please do not bid if you are a company or affiliated to any company ** We have telephony system and developed by inhouse team and looking for billing modules to be integrated to bill our customers. Currently billing is done by carrier itself and we don't deal with it and we just use SIP trunk to do outbound calls and rest is taken care by carrier. Since carrier is doing billing and they don't allow small customers which needs few lines. This is our big problem because we are not able to support small customers. Most of our customers are telemarketers and they have campaigns to run on our Dialers. We need to have a web based billing system so we can support these type of customers. Current software is designed & developed by us and I'm the lead. I have been looking some solutions like ASTPP & jbilling etc to fulfill our requirement. We are on windows platform and may move to Linux in future. Our current configuration related stuff is maintained by web interface using PHP & MySQL database. As for requirement goes, I think what I need is to have a nice interface and bill cycle could be selectable by admin ( weekly , monthly etc) and I'm thinking to use flat call rate ( say 1 cent per minute) and we will have specific campaign ID to bill against and that will have all detailed CDRs. It seems the most simple model if you have previously worked on anything related to billing. I'm already doing some CDR stuff and it can be tweaked anytime based or requirements. I'm looking for someone who has developed or integrated billing software in past and has extensive knowledge in Java or PHP who can closely work with me to finish this out. I would prefer to use some open source project like jbilling or ASTPP etc.. but if you have some other ideas you are most welcome. I can give more details once I get response from you and your skills,experience details. thank you.