4452d530_able___FreeSWITCH___Confluence__Description
[ITU-T Q.850 Code] 20 [SIP Equiv.] 480 [Enumeration] SUBSCRIBER_ABSENT [Cause] subscriber absent [Q.850] [Description] 
This cause value is used when a mobile station has logged off, radio contact is not obtained with a mobile station or if a personal telecommunication user is temporarily not addressable at any user-network interface. Sofia SIP will normally raise USER_NOT_REGISTERED in such situations.
Hangup Cause Code Table - FreeSWITCH - Confluence
Description