Do hospitals have radio communications with air ambulances?

Kind of...

Hospital heliports are "uncontrolled airports" normally, and don't require a clearance from ATC (unless the hospital itself is within positive control airspace because it's near a regular airport). However, the helipads are private "prior permission required" airports, which means the hospital, as the owner of the pad, needs to give permission to land (same way that a helicopter couldn't just land in your back yard unless you gave permission).

The helicopter will be equipped with an FM 2-way comms radio (examples of frequencies here), that is used to talk with a given hospital's flight coordinator or EMS dispatch to coordinate arrivals and departures for logistics purposes, similar to the radio a police helicopter would use to coordinate with dispatch/911.

So if you were just buzzing around in your own helicopter and thought it would be cool to land at the hospital's helipad because you had the urge for a wonderful hospital cafeteria lunch, you'll get a tongue lashing from somebody. To do it legally, you'd have to have the required FM radio and get permission.

There will be a normal aircraft comm radio frequency assigned for VFR advisories when coming and going, like here (where you have a VFR advisory frequency on 119.4 mHz). The pilot will just broadcast intentions on that frequency when arriving and departing, for the benefit of any other helicopters in the area, similar to arriving and departing from any other uncontrolled airport.