i'm thinking here that you should add the MCU as Conferencing Resource in CUCM for Ad Hoc conference calls, or you can setup Randevzous or meet me numbers on the MCU and create a SIP Trunk to MCU with dial plan pointing to your MCU.
The error your getting seems to me that your attempting to register, and CCM is ignoring you and its timing out and not providing a response. So you send REGISTER, and CCM doesn't respond and it fails.
This may get you started, and some other folks on the forums have posted similar issues:
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