Phone Receiving Calls for a specific extension when it is not part of a call queue group

Can you double check who logged in that phone? Can it be that the same user but different phone number (user with multiple phone numbers)?

You wrote "and the phone that is having the issue is not listed in the Call Queue Group". Call queue members are not "phones" (or devices) but extensions. And an extension might have several phone numbers and devices. In this case, I would expect that all of his phones would ring.