[Freeswitch-users] Call duration > 0 while hangup cause is not NORMAL_CLEARING

Hi, I am seeing a lot of calls with hangup cause/Enumeration is not NORMAL_CLEARING while the calls still have a valid duration and answered time. For a typical billing scenario, calls are billed if (answered time) billsec>0 and hangup cause = NORMAL_CLEARING. In what cases the enumeration would be other than normal and while the calls would be still a valid answered call? Thanks