The CCM has no awareness of the H323 device's capabilities, so it will keep sending calls to it until it gets some kind of indication that it can no longer process the calls.
If the calls are intermittantly failing like this, you can either go through the CCM traces to see what clearing reason the gateway is sending back, or the other option is to run wireshark to capture the calls between the CCM -- GSM device, then use the VOIP calls analayse feature to detail the call flow.