Illinois has a mutual aid pact between fire departments called the Mutual Aid Box Alarm System (MBAS). 1,175 of the state’s 1,246 fire departments belong to MABAS plus the cities of St Louis and Milwaukee. They have pre-defined protocols and arrangements for dealing with statewide emergency declarations as well as day-to-day assistance requests.
For example, during a 2004 downtown high-rise fire in Chicago, 22 suburban fire departments moved into Chicago fire houses to replace the units handling the downtown fire and answered service calls in the city.
Just a quick note about the edited OP: because vBulletin simply ignores anything within a post in brackets if it isn’t a vB command, the mouseover displays the text of OP without the inserted characters. I don’t mind, but some others might.
Mutual aid can help but even without it (just within the city jurisdiction for example) when one fire station is emptied because everybody goes out on a big call, the dispatchers will get staffing and sometimes equipment from nearby stations over to the empty one to maintain coverage. It happens as a matter of course, standard procedure rather than waiting until the station is literally empty or waiting until shift change.
Also, most Fire, EMT and dispatchers that I knew when I worked for a county 911 agency lived and breathed emergency services so they all had scanners at home and radios in their car. They always knew what was going on at all times even when off duty.
That was true of my relatives who worked in public safety, too.
And if some big problem happened when they were off duty, they would just go in to work to see if they were needed, before the dispatcher ever got around to calling them in.
I think this is common – I remember that many of the 9/11 rescuers had been off dutym, but came in to help.