Sorry, I wasn't clear in my request: what I meant to say is what do YOU (as the street level provider) get to do at the MCI training? and does your entire department participate, or only specified units? Do you recall an entire off duty shift? What if its your regular work day?
The last MCI drill I attended was at Newark Airport; I don't remember the details (I think there was an issue in the terminal and on the plane, because we had two operations running simultaneously, one ramp side, and one street side), but there were definitely more volunteer ambulances than career ones in the staging area. Most career agencies sent one ambulance, and their Special Operations Vehicles (we sent several white shirts, all of our off duty SOG personnel, several mass casualty units, and one ambulance), while many volunteer agencies brought two ambulances with 3 people per truck. So if you were working that day, or already scheduled for your other job, you didn't get to participate.
I know many FD (like
@Jim37F stated) allow their units to to OOS for training, but I don't know many EMS agencies that do this; this barely have enough units to cover the call volume as is, so taking one or two out of the system would cause a further strain on existing resources.