Fire Based EMS would be the answer to it spawning EMS calls. This mission is not the only one that spawns that needs and EMS vehicle but does not require it until after someone gets there and the "we need an ambulance" appears
This mission was for the UK game so we dont to my personal knowledge have combined fire/ambulance appliances like the US does.
I think this is just a bug where they meant to select ambulance station to spawn said mission but put fire.
The station that spawned it does not have an ambulance expansion attached.