Depends on what you want to do. If this is a PSTN call, you cannot block a specific call at the IP phone level. In this case, you would need to use an H323 configuration where you can configure a blacklist and block calls based on caller ID.
If you want to block User A from calling User B, you would need to leverage partition and CSS setup to do this - however, this is rarely done for one specific caller.
Hailey
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