The amount of calls you get is determined on highest amount of stations (most Police or Fire etc) +1, so i have 314 fire stations so I get 315 calls. If you are a new player I strongly advise you to get fire stations first.
I always recommend people to get 13 fire stations then 3 ambulance stations and then go on to either police or continue fire or EMS as this way you will get some of the highest paying calls such as building collapses etc.
a little bit offtopic
how much cost your next station?