To start with you can recreate the issue and take parallel packet captures from one of the IP phones , TFTP server and the primary cucm server to which it is trying to register. This will give an idea if all the packets exchanged between CUCM and IP phone are making it to respective destinations ( filter the SIP , SCCP , TFTP traffic to begin with ). You can set up a capture on ASA as well to see if it is allowing the signaling between CUCM and IP phone in both directions and determine which part of the topology has a problem.
Manish
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