Voice Inversion is 1970's technology that was pretty much the only way to go unless you had VERY deep pockets back then. I was a deputy from '78 - 91, and IIRC, the device was an outboard box that the mic and speaker hooked up to and then the box was connected to the control head. Flip a switch and you were "secure".
The SO looked at getting them, and the salesman was really pushing the sales pitch that no one could monitor the radio traffic. He set up a demo where he had a unit and hooked it up to one of the patrol cars temporarially so he could show us how great it was. I brought in my xtal scanner I had at home and one of those DNE descramblers I had hooked up to my Regency programmable. Of course it decoded the traffic and that pretty much killed his sale.
AFAIK, there may be a few agencies left using them (probably small, rural departments), but they are pretty much gone in the public service area. I know there is a lot of bootleg activity using the PRC imports that have the "scrambler" function on the MURS frequencies and other itinerant channels.