Trying to EDIT users and while editing, the SAVE button is grayed out for some users.
CloudSOC
This is caused by a wrong phone number format, where we have country code (prefix) included at the beginning of the full phone number, or the phone number is stored using another unsupported format.
Enhanced ADSync feature currently doesn't validate phone numbers before they are submitted to CloudSOC.
Edit phone number and remove it or correct it in a way where you choose country code (prefix) from drop down list and have only phone number in field.
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Reference: Create CloudSOC Users and Groups