The City of Philadelphia had a Centrex for as long as I can remember, MUnicipal 6-9700. During the bicennial they changed it to MU 6-1776. IIRC they used the whole MU 6 block. Despite that, not all phones were on Centrex and outsiders had to go through the City Hall operator (a 24 position 608 switchboard split into two sides of 12 positions each). The non-Centrex extensions had five digits and access codes were required to reach between five and four digit extensions.
As to city offices in the field, such as libraries, local police stations, playgrounds, etc., some had MU 6 numbers but some had local neighborhood phone numbers. I could never figure out a pattern.
In addition, the City had a full separate private PABX (private dial exchange) that was old. It mostly served police and fire stations and street call boxes, and hospital emergency room desks, not so much city offices. Its phones were immediately recognizable -- old AE sets with the nickel ornamentation and a ratty brown fabric coiled cord. That dated from when before radios and police and firemen used street callboxes to call in. (Street callboxes were not extended to newer postwar neighborhoods). I don't know when that system was removed from service -- I've seen in use in the 1970s.
For street fire alarms, the city used call boxes. When the lever was pulled, a clockwork would generate a coded signal denoting the firebox. These were removed a few years ago. As kids, we were taught to know where the nearest callbox was to our homes and to wait there to direct the arriving firemen to the fire. They date from a time people didn't have phones or speak English, and lasted a lot longer than they needed to given universal telephone service.
The City was slow converting to Touch Tone. In the early 1990s city telephone sets were still classic hardwired 500 sets with a nice printed MUnicipal 6- number card. Finally they went to Touch Tone, but plain 2500 sets. They added 685 as another Centrex to 686. The phone directory lists a lot more prefixes to reach various city offices today; I don't know how it is set up.
I've read City Hall (an ancient huge building) originally had DC power and stayed with it until 1954, which was pretty late. In the early days of electric utilities, some places put out DC instead of AC. (There was a debate in the industry which was better and AC won out). Old catalogs of the 1920s and 1930s list products in both AC and DC versions.
[TELECOM Digest Editor's Note: City of Chicago used DC power (rather than AC) -- at least in the downtown area -- until sometime around 1930. That's at least one reason why there were so many WUTCO clocks everywhere, instead of 'regular' wall clocks. Clocks cannot run on direct current; they require alternating current at 60 cycles.The old city hall (Chicago) PBX on RANdolph 6-8000 had a _20_ position switchboard, broken in two parts also -- 10 positions in a rank along one wall, the other 10 positions along another wall. The only bigger switchboard I ever saw was the one at University of Chicago when I was working there, 1958-61 or so. It was 28 positions, but divided into three ranks of 13-13-2, a rank against each wall and the smaller group along a third wall. There were three groups of directory listed numbers; main campus was MIDway-3-0800 (with 2xxxx, 3xxx, and 4xxx extensions), hospitals were MUseum 4-6100 (5xxx, 6xxx) and the Computation Center and Fermi Institute, NORmal-7-4700 (8xxx). From 6 AM to 1 AM next day, they usually had at least 20 operators on duty, but in staggered shifts (a couple came in at 6 AM, a few more at 7 AM, etc.) By 11 PM when I got there, two or three women were left. They had all split by about 1 AM leaving me alone until when the day shift started coming in a few hours later. Volume of traffic was such at that time of night it did not warrant any more help. When something 'buzzed' (buzzers were turned off during the day, [just work from the light blinking at you] but turned on overnight, sort of a loud sound) I just walked over to the rank in question, plugged in my headset and did business. Along the fourth wall were desks for the chief operator and the clerk and the teletype machines and their clerk/caretaker.
The teletype machne hardwired direct to the 'Kenwood Bell' central office (so named for the Bell central office at 61st Street and Kenwood Avenue which served our lines) was used for time and chargs coming in for long distance calls. The other one was hardwired to Western Union for incoming messages. Another big board was the one at Sears, Roebuck on State Street downtown (WABash-2-4600) which was a five position board, serving the department store and the credit offices upstairs. That one was busy also, it literally rocked around the clock; our at UC at least slowed down considerably during overnight hours. PAT]