[Serusers] accounting accuracy

Hi list, I'm having problem with my accounting, I have a SIP server and I'm using a commercial gateway provider where I can terminate calls. Unfortunately, there's a big discrepancy on the number of minutes I see on my billing system, compared to the billing of the gateway provider. Every month there's always almost a thousand or more minutes diiference. For my billing system, i used the following fields to get the start of call, sip_method=INVITE sip_status=200 and username='username of caller', then to get the end of call, sip_method sip_callid totag and fromtag. are these correct? Am I doing something wrong there? Also someone told me I need a B2BUA for the calls to be accurate, is this true?Hope anyone can help me. TIA Regards Nhadie