Who the hell would design a system involving 911 calls with a limit that could be reached in a couple years of calls? I can't imagine how I would go about being stupid enough to not understand the nature of the requirement to not have 911 calls fail. This isn't cobol with a fixed field limit of N digits someone specified in 1964.

40 million, according to the article at the moment.

Edit: Thinking it might be because they limited the variable to a specific number of bits, I checked to see if any maxed out around that number. 25 bits max out at 33,554,431 and 26 bits at 67,108,863.

Haven't they ever heard of a UUID? Or if they are worried about potential collisions at least make the number 128bit (since you have an obscenely large max value to increment to).