Hi @fang su ,
Do you mean you only want to hide the phone number of a specific mail contact for Outlook users in your organization?
If this is the case, I am afraid currently there's no direct way to do this from either the Exchange server side or the Outlook client side.
Given this, a possible workaround I can think of is to enter the phone number into another field which doesn't display in Outlook instead, such as the Notes field:
Based on my test, information in the Notes field doesn't show up in Outlook contact card:
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