I need to validate an alteration

Summary:i need to validate an alteration only on Phone and email

Content (required):I have an approval flow on my environment for new record it works well, but i have a problem to update the records,

when a record is updated i need to validate if the fields Phone and/or email was changed, because the user if he is not an approver need to mark a flag to trigger a workflow who notify the approver to validate the change, but i need to prevent the user from saving the record without mark the flag, i tried to use the isAttributeChanged function, but the result doen't work as expected, i tried to reffer the whole collection of phone or email, but the error message i created to warn that the flag was not checked is appearing even