I see medical emergency helicopters taking off and landing several times a day from the hospital near my office. As I understand from an earlier question, they are probably flying VFR, below controlled airspace and likely without ADS-B.
I would assume, however, that these flights are in touch with the local ATC, and they are certainly announcing their presence on the locally monitored channel(s). Is there a particular set of call signs or other indicators that are used by these flights to indicate to other traffic that, while this flight itself is not in any sort of mayday situation, it needs priority handling and that everyone else needs to get out of the way as lives depend on the speed of this particular flight?
I am asking particularly for the USA/FAA, but would be interested to know if there are differences in other countries around the world.