If the response is within Mecklenburg, Cabarrus, Union, Stanly, Gastonia, or Mooresville (where the UASI radio system is the primary means of communications) you will find MedCenter Air on CMC-LZ-1 or CMC-LZ-2.
If it is outside of the counties mentioned above or a different helicopter service they can be found on VIPER LZ Central or LZ West. There is the slight chance they operate on state fire or a specific county conventional frequency as well in some cases.
I rarely monitor VIPER or UASI, and can add nothing on that front, however a few months ago I heard a chopper nearby (I was in Stanly Co) and went to signal stalker mode. Caught 3 VHF freqs, IIRC all 150's with PL's. Each time the pilot was trying, apparently with no joy, to contact ground about the LZ situation. I'll have to find my notes for exact freqs, but they were all specific to stanly county or common statewide medical related, tho not Med Helo specific.
Only speculating here, but I got the impression that there was a failure with the SOP's,such as using VIPER or UASI TG's on that occasion. So you may have times where you have to go outside a trunked system even in areas where that is uncommon.
Oh, also I can't remember if it was MedCenter Air specifically, possibly another outfit, which may or may not invalidate the above.