When a new phone is added, in the phone configuration page, it will be assocaited with a calling search space. While in the next page which is "directory number configuration", the directory number again is associated with the calling search space. Why both the phone itself and its directory numbers both need to be assicated with calling search space? Is that possible just do the phone, not the directory or vice versa?
They combine to create just one, read this for full explanation
Calling Privileges in Unified CM
I would strongly suggest you to start reading CUCM material (SRND, config guide, features and services guides, etc)
Scenarios of when to use device, when to use line, both, etc are also discussed in the link. I would recommend you to fully read that chapter.
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