4452d530_able___FreeSWITCH___Confluence__Description
[ITU-T Q.850 Code] 18 [SIP Equiv.] 408 [Enumeration] NO_USER_RESPONSE [Cause] no user responding [Q.850] [Description] 
This cause is used when a called party does not respond to a call establishment message with either an alerting or connect indication within the prescribed period of time allocated.
Hangup Cause Code Table - FreeSWITCH - Confluence
Description