I would bet you're right about having established plans for mutual aid. I moved here from the Chicago area about 10 years ago. While the City of Chicago handled it's own alarms, even at the borders with the close-in suburbs, the 'burbs came up with what is called MABAS, the mutual aid box alarm system. Under this system each suburban community handled the initial response to their incidents, help for larger fires or other emergencies came from the surrounding communities on a preplanned basis. The plan even provided for further distant communities to change quarters (move up or fill in) so that wide areas weren't left without a degree of protection for other emergencies.
After reading your response, I googled "mutual aid box alarm system" and found several interesting websites including a nice PowerPoint presentation.
This MABAS system must be working well. Before I moved it had spread across northern Illinois and the very south eastern part of Wisconsin. I found the following quote on one of the websites: "MABAS is a mutual aid organization that has been in existence since the late 1960s. Heavily rooted throughout northern Illinois , MABAS now includes over 750 member fire departments organized within 75 divisions. MABAS first came to Wisconsin in 1987 with the joining of the Village of Pleasant Prairie in Kenosha County . MABAS divisions geographically span the states of Illinois , Wisconsin , Indiana , Missouri , Michigan , and the other states, i.e. Minnesota , Iowa , and Ohio , are considering joining. There are 16 established Wisconsin divisions (as of May 2008) who share MABAS with the 62 divisions established in Illinois .
"MABAS includes over 25,000+ firefighters and daily staffed emergency response units including more than 950 fire stations, 1100+ engine companies, 375+ ladder trucks, 800+ ambulances (mostly paramedic capable), 150+ heavy rescue squads, 135+ light rescue squads, and 275+ water tankers. Fire/ EMS reserve (back-up) units account for more than 600 additional emergency vehicles. MABAS also offers specialized operations teams for hazardous materials (HAZMAT), underwater rescue/recovery (DIVE) and above grade/below grade, trench and building collapse rescues, a.k.a. Technical Rescue Teams ( TRT ). An additional element of resource are the certified fire investigators which can be "packaged" as teams for larger incidents requiring complicated and time-consuming efforts for any single agency."
Thought you might find that interesting.