The best way to achieve what you want is to have two very low slip motors running a set of siren choppers and two VFDs that are capable of alternating the frequencies quickly. It would be very challenging to alter pitch based on ports while the siren is in motion.
IIRC Sentry's S.T.A.R. system uses an encrypted DTMF string somewhere around 20 or 30 digits long. I've only heard it once in a video, its quite unusual sounding.