I am currently building realistically, and the one of the few departments in the state that has helicopters is the state police. However the state police assists all jurisdictions as need. I find it only logical that you be allowed to tie a building to multiple dispatch centers (rather than just disabling the create your own coverage area option). Which would allow you o meet the call requirements without having to A) place a station per dispatch center or B) disabling the create your own coverage area option which makes the smaller areas more prone to massive calls.
TLDR: Make it so that a station can be tied to more than one dispatch center.