4452d530_able___FreeSWITCH___Confluence__Description
[ITU-T Q.850 Code] 81 [Enumeration] INVALID_CALL_REFERENCE [Cause] invalid call reference value [Q.850] [Description] 
This cause indicates that the equipment sending this cause has received a message with a call reference which is not currently in use on the user-network interface.
Hangup Cause Code Table - FreeSWITCH - Confluence
Description