That is the way it is defined - sip/13579
Yet, there are no calls to this device, and the queue can not distribute the call. However, as a test, if I dial this sip/13579 from another extension (before or after trying to queue a call), there is no problem, the call is answered. The sip/13579 is not being used, the queue show command reports it as not in use, and yet, the app_queue fails delivering the call to it.