4452d530_able___FreeSWITCH___Confluence__Description
[ITU-T Q.850 Code] 42 [SIP Equiv.] 503 [Enumeration] SWITCH_CONGESTION [Cause] switching equipment congestion [Q.850] [Description] 
This cause indicates that the switching equipment generating this cause is experiencing a period of high traffic.
Hangup Cause Code Table - FreeSWITCH - Confluence
Description