[Serusers] accounting

Hi All, Am I correct on using these for my accounting, first, for the start of call, i used the username, sip_method="INVITE" sip_status="200" callid, fromtag, totag and time. for the end of call, i used sip_method="BYE" sip_status="200" callid, fromtag, totag and time. My problem is that sometimes the gateway is sending sip_status="487" maybe the call got disconnected. but we are already getting billed for that by the gateway provider, because to them we already used up some of their minutes. I tried a sample script that would make use of sip_status=200 or sip_status=487 to get my end call time. but i'm seeing calls that are 512+ mins, maybe the sip_status=487 from the gateway returned very late, what could be the problem on that? I will truly appreciate any help thank you. Regards, Ronald