This reason for the DN being greyed out is typically because call-control is not established. Verify the following:-
Make sure that the user ID that you use for the attendant console application user is the same as the value in the JTAPI username field in the Service Parameters Configuration window for the Cisco CallManager Attendant Console Server service.
If you are using device authentication make sure that the user ID that you use for the device authentication user is the same as the value in the ACDeviceAuthentication field in the Service Parameters Configuration window for the Cisco CallManager Attendant Console Server service.
Make sure that you check the Allow Control of Device from CTI check box on the Phone Configuration window (Device > Phone) for each attendant phone.
The system enables this field by default. If this check box is not checked for the attendant phone, call control does not come up for the attendant console
Make sure that you associate the attendant devices with the attendant console device authorization user that you configured in the Application User Configuration window in Cisco Unified CallManager Administration.
Hope this helps
Regards
Allan.