All Mercy air ships are dispatched by "LifeComm" in the midwest (Topeka, KS I think). They are dispatched by satellite radio communications in many areas, but in Southern California and Nevada they have radio channels that are linked to their dispatch, likely via phone line or over internet.
In SoCal, they can be heard on 131.675 and 155.22 (in the desert east and north of LA)
In Nevada, they can be heard on 453.875 on a repeater.
I know a pilot for Mercy, so I emailed him and will post his response once I get it regarding Mercy Air 20.
I just heard Mercy air 20 doing radio checks on cmd 6 tone 4 witch is south Monterey county. I'm guessing they are going to be station in south Monterey county now due to how long it takes for cal star 5 and 7 to get to Big Sur,lockwood,highway 25 and highway 198 area.
Mercy Air 20 is a private air ambulance service, contracted to the US Army at Fort Hunter Liggett. They are dispatched through Fort Hunter Liggett dispatch center. Through an arrangement with the military, Mercy Air 20 acts as a mutual aid county resource to Monterey County. Requests for service off-post are made through FireCom which are then relayed to Fort Hunter Liggett dispatch.