I just registered a 7945, 502G, 7960 and a 7970 to a UC540,
But I am unable to get a 6921 registered.
I tried it automatically by “Adding to voice network…” from within the CCA (CCA sees it), and that did not work.
I also tried adding it manually by adding the MAC address.
It was registered a few weeks ago, and seems that it lost its registration (this is a lab Environment)
I do have the 69xx phone load, loaded into the UC540.
I've had bad experiences with manual additions to the UC in combo with CCA. Try deleting all manual entries of the phone and the automatic entries as well and try adding it again via CCA.
Broken phones have to be deleted completely instead of changing the MAC-address in the ephone with the replacement MAC-address of the new phone. Doing it otherwise would result in a unusable phone until everything was cleaned up..
Havent figured out what the mechanics exactly are but deleting and and adding via CCA is the safest option.