These are popular in many gated subdivisions around Tennessee cities.
Usually these gates use an audible overide sensor keyed to the yelp sound from the siren. However, some services found they needed a siren manf. particular yelp pattern (i.e., Federal instead of SVP). The siren yelp can be calibrated, or a tone recorded at the dispatch center can be retransmitted and played over the PA system.
The dispatcher might also obtain any key code from the caller so that may be entered to allow access.