Mercy Air 20
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.