The maximum number of your concurrently available calls is determined by the building type with the highest amount +1.
Example: Joe has 20 fire stations and 19 police stations. That means he has a maximum amount of 21 missions a every one time. Now Joe builds two more police stations. Now he has a maximum amount of 22 missions at every one time.
So to get more calls you have to build more stations of the type of station you have the most of.