I am having an issue and I found the resolution I think in the troubleshooting guide.
There are no in-service Call Servers. The Call Server may take itself out of service when ICMresponse times get too long.
Within the Operations Console, go to Device Management > Cisco Unified CallManager. Goto Engine > Engine Configuration. Change New Call Throughput Upper Threshold andCall Event Throughput Upper Threshold values to 30000.
However I can't find this setting or location in CVP 4.0(2), I think the documentation is a bit off, any ideas where I can find this setting?