4452d530_able___FreeSWITCH___Confluence__Description
[ITU-T Q.850 Code] 17 [SIP Equiv.] 486 [Enumeration] USER_BUSY [Cause] user busy [Q.850] [Description] 
This cause is used to indicate that the called party is unable to accept another call because the user busy condition has been encountered. This cause value may be generated by the called user or by the network. In the case of user determined user busy it is noted that the user equipment is compatible with the call.
Hangup Cause Code Table - FreeSWITCH - Confluence
Description