4452d530_able___FreeSWITCH___Confluence__Description
[ITU-T Q.850 Code] 44 [SIP Equiv.] 503 [Enumeration] REQUESTED_CHAN_UNAVAIL [Cause] requested circuit/channel not available [Q.850] [Description] 
This cause is returned when the other side of the interface cannot provide the circuit or channel indicated by the requesting entity.
Hangup Cause Code Table - FreeSWITCH - Confluence
Description