Yeah, the most ambulances are stationed with the fire department. The doctor is at the hospital, at work, and if needed, a car from the fire station goes to the hospital and fetches him while the ambulance is already on the way to the scene, making the responding time as short as possible
You dont need to respond two units to every call. People shot by your police officers, people who only fell, people injured by gas or toxics or stuff who are only slightly "damaged" may be transported in one of the RTWs (ambulance) or even by the KTW (ambulance waggon used for transports and low level emergencies)