4452d530_able___FreeSWITCH___Confluence__Description
[ITU-T Q.850 Code] 19 [SIP Equiv.] 480 [Enumeration] NO_ANSWER [Cause] no answer from user (user alerted) [Q.850] [Description] 
This cause is used when the called party has been alerted but does not respond with a connect indication within a prescribed period of time. Note - This cause is not necessarily generated by Q.931 procedures but may be generated by internal network timers.
Hangup Cause Code Table - FreeSWITCH - Confluence
Description