I apologize, but I completely missed this question until today.
The Network Congestion would be a message the ATLAS gets from the Telco (network).
Generally it means exactly how it sounds: the network is congested and can't process the call through.
It is an ISDN cause code generated by something downstream of the local PRI indicating that there are no available circuits to route the call. On a large carrier this may result in an intercept message along the lines of "All circuits are busy now....". On smaller systems it usually results in a reorder (fast busy).
Here's a list. Most likely cause 42 but could be 38. http://www.dialogic.com/webhelp/img1010/10.5.2/webhelp/General_Reference/def_sip-ss7_cc.htm