Xact Dialer "FAILED" calls

A lot of calls generated by Xact Dialer are marked as “FAILED” on CDR reports.
Campaign is set to use TWO concurrent calls over FOUR channel available (gateway is a cisco router using a pair of dual BRI VIC)
All FAILED calls have the “CallerID” value as UNKNOWN ( while the regular ones has the same number as the Outgoing CallerID) and all “App” as CONGESTION
Which way should I start to debug the issue ?
Is it usually a provider, gateway or pbx side fault ?