Can't I use geroepen?
No, it would change the meaning. Instead of calling an ambulance (with a phone), you'd say you have been shouting to the ambulance.
CALL THE AMBULANCE MISTER AMBULANCE!
When should i use de ambulance and de ziekenwagen?
They are interchangeable. People in my area always say 'ambulance', but you can choose whatever you want. ;)
Thank you for the answer. :)