One of the first places to get 911 was Springfield, Mass. At least they had it by 1972. This was possible in part because they had a step-by-step central office 73- (REpublic)-- including 739- --with absorbing first selectors, so that you could dial (from other SXS lines) just the last 5 digits 9-xxxx (or 911 instead of 739-11). Since SXS switched the call on each digit, you didn't have to have 7 digits.

So I guess anyone who had 739-11xx numbers had to get number changes in order to assign 911 (or 9-11 or 739-11) for emergency calls....There was also a crossbar c.o. (STate 1- or 781-).