4452d530_able___FreeSWITCH___Confluence__Description
[ITU-T Q.850 Code] 21 [SIP Equiv.] 603 [Enumeration] CALL_REJECTED [Cause] call rejected [Q.850] [Description] 
This cause indicates that the equipment sending this cause does not wish to accept this call, although it could have accepted the call because the equipment sending this cause is neither busy nor incompatible. The network may also generate this cause, indicating that the call was cleared due to a supplementary service constraint. The diagnostic field may contain additional information about the supplementary service and reason for rejection.
Hangup Cause Code Table - FreeSWITCH - Confluence
Description