Have you associated the CSF device to the end user as "controlled device"?
CUCIMOC follows couple steps to determine the device name:
1) CUCIMOC talks to CCMCIP service on CUCM to retrieve a list of devices associated with the user.
2) From the list, CUCIMOC picks the device with device type = CSF.
3) CUCIMOC tries to register this device with CUCM as any regular phone, which means, it goes through TFTP download, registration, etc.
Any failure in the steps above will fail the CSF registration.
Michael
http://htluo.blogspot.com