The Help article for the cmaDeviceAlerts shows the record and field and then looking up the definition, also in Help, finds that it is the Total number of times that a call was attempted when no voice channel was available last interval. The alert is triggered when this is greater than 0.
Prognosis gets the stats from the VG / H.323 gateway by doing a periodic SNMP query of the dialCtlPeerStatsTable as defined in DIAL-CONTROL-MIB, and then takes a delta from the stats of the previous query.
You might ask Cisco TAC why the figures are changing in that SNMP table on the gateway if the 12 T-1s aren't all full.
The more precise info you'll need to enquire to Cisco TAC with is that the Prognosis alert is triggered when this Cisco metric changes.
"dialCtlPeerStatsRefuseCalls" in the "dialCtlPeerStatsTable" SNMP table from the gateway and the Cisco description of this metric is:
"Number of calls from this peer refused since system startup."
See the Cisco DIAL-CONTROL-MIB for more details.