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The user hasn't been added to the Call Centre
If the user has the wrong Extension No. assigned to them then Salesforce will perform a screen pop when the real extension owners phone rings.
i.e. Your phone rings but my screen pops.
If the user's account does not specify an Extension No. then Salesforce will not pop.
If the user moves desks (or changes their Extension No.) they will need to update their extension no. in Saleforce.
You can check if a users Extension No. is set correctly from the Noojee 'Phone' utility bar page.