Single inversion generally reverses the highs and lows in your voice,then injects a variable or stepped null tone. I use to build these for people back when they were still legal.
I still have a few in various states of construction.
Funny story. Many years ago (before the terrorist mania) our local PD purchased an elaborate scrambling system for all their cars. It was very complex, as they had to change the "code" every day so all units would be on the same setting. I used to hang around the station, and knew most of the officers. One night a Sergeant was trying to impress me with their new communications security device, and was bragging about how secure their new system was. I said, "really"?, how about I go to my car and you go inside and transmit a scrambled message to one of your cars? He did, and when he came out I told him word for word what he had just said. His face dropped. I think the chief's did too after learning that his expensive system could be beaten by a $35 descrambler with a tunable pitch. It plugged into the external headphone jack on my scanner, and had its own speaker. Didn't matter what code was selected. I purchased the descrambler from a guy in Hot Springs and used it for many years.