The Avaya CDR format time/duration stamp has positions for hour and minute of call hour, minutes, and tenths of duration in the format of HHMMHMMT. If a call exceeds 9 hours 59 minutes and 9 tenths (or 9 horus 99 minutes and 9 thenths, I forget which) the PBX issues a call record of HHMM9999 or HHMM95999 for the call duration up to that point, and then an additional call for the 'balance' of the call duration until the end of the call. The call accounting is expected to be able to add them together for the actual duration, but many do not. If the call extends for more than one 10 hour period, you will have multiple 9999 duration records, and the one 'ending' record.
This is usually caused by a hung trunk of some kind--caller dials outside number, gets autoattending, decides to abandon call, hookswitches, dials internal extension and hangs up. The trunk is is on-hold, and the far-end didn't disconnect the trunk.