The first sirens I heard with a 'phaser' tone were Carson / Southern VP electronic sirens in the early to mid 1980s. They were the old SA400 series with the low-pitched tones."
I recall SVP sirens in our area were the first with Phaser tones. Many of our State agencies still use a Federal PA300 with High/Low. I also think it is better when responding through crowds than Manual Wail, Yelp or an Air Horn. In my observations, using Yelp, crowds seem to scatter all over the place, some into your direction of intended travel. With the air horn, they turn around and look with an expression like - you honking at ME? High/Low - they know it is an emergency vehicle, and seem to part pathways. Now if you also had a true air horn, like a Fiamm or a Grover, you could move recalcitrant persons, and expedite the bowel functions of some.
A research article published in (Montreal or Toronto?) Canada also showed that where Hi/Low was used, vehicle operators and dispatchers could better understand each other in radio communications. The tones cut through voice communications less than some other siren tones. Two dollars of microchips installed in the mobile radio microphones could cut out pure tone Hi/Low so the dispatchers couldn't even hear the siren in the background.
However, some states (California and others) do not recognize High/Low as a siren. IMO
- It still has a use.