There are a couple of reasons why this might be occuring that jump to my mind:
- The 7975 was attached to a different cluster, thus it has an ITL file that does not match your CUCM cluster
- The CUCM cluster has DNS enabled and your phone cannot resolve the server's hostname
- Your 7975 has a pre-8.5(2) code and needs to be brought up to that version first before you install firmware 9.0
- It is rare nowadays, but there was an bug awhile ago that affected shipping phones where they could not upgrade their firmware unless they were on the same VLAN as the CUCM
For the case of the ITL file, do a factory reset of the 7975 and let it download a new ITL file.
Also check DNS, ensure the phone has a domain name and DNS servers that can resolve the hostname of the CUCM. If there isn't an A and PTR record for each CUCM in the cluster, create them.
Look at the firmware on the 7975, if it is running firmware prior to 8.5(2) you must upgrade to 8.5(2) and then you can upgrade past that version.
If all else fails, put the phone on the same network/VLAN as the CUCM and see if that gets it working.
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