Back in the day, we had only landline phones and these, of course, involved actual, physical wires that ran all over town. When there weren’t a lot of people with phones, your assigned phone number might have only 4 or 5 digits….the first number my family had was 6-1635, for example. As the population grew, the phone numbers had to get longer to accommodate all the new phones, and instead of human operators in a big “central office” downtown who made the connections, mechanical “switches” came into use, and these were basically big pieces of very cool equipment that figured out how to connect you to the phone number you had dialed in.

Yeah, remember the old timey rotary phones? The ones with the dial on the front? So, in a simplified way, what would happen is, if, for example, someone dialed my phone number 6-1635, the switch would have to figure out “ok, I have to find the main circuit where the phone numbers all start with 6. Then I have to find the next subcircuit that has the 6-1 numbers, then the next subcircuit with 6-16’s, then the 6-163’s,” and then locate my particular phone on that last circuit.

The switches were big pieces of electromechanical equipment, and were installed out in the. neighborhoods, and phone numbers were organized assigned geographically because of all the wiring and equipment involved—for a very simple example, phone numbers starting with 1 might be assigned to the north part of town, 2’s to the southside, 3’s to the east, and 4’s to the west. When someone told you their phone number, you had a rough idea of where they lived in town.

As the population grew, so did the phone numbers. Ours went from 6-1635 to 626-1635. To help folks remember the numbers, mnemonics were assigned—that’s what the letters were for of phone dials. For example, our “exchange” became known as “Market”—because, looking at the phone dial, the letter “m” was assigned to 6, and “a” to 1, and “Market” was a word that could be made that started with “ma.” Just a couple of the other exchanges in town were “Pershing” (PE, 73) and “Edison” (ED, 33). So, if I told you my number was “Market 6-1635,” you knew I lived somewhere on the north side of town; “Pershing” was on the west side, etc, etc.

Eventually, the phone company realized that people were able to actually remember a string of digits, and dropped using the mnemonics. (And eventually, our number went from 7 to 10 digits, so that the area code was required not just for long-distance calls, but for local calls, too.)

The “Klondike 5-“ seven-digit number is a remnant of that system. Remember the song “Jenny 867-5309”? Whenever real phone numbers got used in the media like that, the poor people who actually had those numbers IRL would get zillions of prank and spam calls. So writers started using fictional phone numbers…the exchange “555” (KL 5-) is not used in the US, and so if someone tried to call a “Klondike 5-“ number, they get nothing but the “this number is not in service” message.