Try these:
155.3400 Statewide ER Net
463.1750 Statewide MED Net
155.3850 North MS Medical Center Ops
Also pay close attention to the local Airport freqs and ARTCC..... As the Helo will have to get clearance for flights through various airspaces.
This air ambulance is nowhere near as organized as SEMAAD's or UMC's systems. At one point there were 4 in usage in MS, SEMAAD, UMC's, North MS's, and NWMAAD. I had thought that both North MS's, and NWMAAD's had flopped untill recently.