Dispatchers
To answer Shawn, most HEMS dispatch centres in the US are staffed by the "customer".
Except for those large 135 operators with a wide fleet of "community based" helicopters (as in PHI, AMC and AEL) where they have their own dispatch centres staffed by specialist and where there are pilots in a "flight supervisor" capacity.
Here's the normal process:
1) A 911 call is received requesting medical EMS
2) depending on the severity or mechanisms at least one (usually the best PRd) EMS program is extended a phone call and a helicopter is put on STBY or launched.
3) Some programs scan the airwaves with their radios and launch on their initiative (jump the call).
4) The race against time... and competitors is on...