[Serusers] Call forwarding

Hello, I have a ser and asterisk setup. Users register on ser and all the national and international calls go through asterisk. Asterisk handles the billing part for long distance calls. Now I got an issue with call forwarding. Here is the situation. Useragent A calls Usergent B. User B has setup the call the call forwarding to his cell phone. So when user A calls user B, ser check that user B has set the forwarding and send the call to Asterisk box. Asterisk box sees user A in the callerID and bills user A for that call. We want user B to be billed for that call, cause he is forwarding the calls to his cell phone (long distance number). How can I achieve this. Thank you, -Jai