Hello!,
So I was thinking of some ideas and I came up with this:
There could be a Ambulance Expansion to the Hospitals, when added there will be a call radius just like the ambulance station. Not only is this realistic, but it's very reasonable. I personally don't see any issue with this. You'll still need Ambulance stations to have more than 3 ambulances and to have fly cars and mass casualty units etc.