In Ghostbusters 2, on the side of Ecto-1, the number displayed is “JL5-2020”.
What is JL5? Was/is that a NYC phone system design, or something used in the 80s during the era of landlines?
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First we have to cast our minds back in time to when there were no overlays, which means if you were calling a number within the same area code as you, you could omit the area code. So you could dial just 555-2020 and the phone system would attempt to connect you to that number in your area code. In New York City, this would have been 212, so dialing 555-2020 back then from within NYC would do the same as dialing 212-555-2020 did and does.
Next, we go even further back in time. When phones were newer, there weren’t as many numbers, and each town or area could have its own exchange (the middle three numbers), and there were plenty of available exchange numbers to choose from. So to help with memorization, exchanges would be chosen so that the letters that mapped to the first two numbers were a mnemonic for the town or area. Sometimes in movies or TV, you might hear someone talk about calling “Klondike 5-1212”, or “Transylvania 6-5000”. Those would represent KL5-1212 and TR6-5000, respectively.
What you did is looked at the holes in the dial where the numbers and little letters are, and you dialed the number corresponding to the letter. So for KL5-1212, you dial 555-1212, etc. That means JL5-2020 is a phone number of 212-555-2020. In the USA, the 555 exchange is a dummy that goes nowhere, which is why so many fictional phone numbers have a 555 exchange.
It's just a fake phone number.
If you dialled it, it would be 555-2020 and "555" numbers in the US were traditionally "fake" numbers for TV and movies.
The telephone number prefix 555 is a central office code in the North American Numbering Plan, used as the leading part of a group of 10,000 telephone numbers, 555-XXXX, in each numbering plan area (NPA). It has traditionally been used only for the provision of directory assistance, when dialing NPA-555-1212.
The central office code is also used for fictitious telephone numbers in North American television shows, films, video games, and other media in order to prevent practical jokers and curious callers from bothering telephone subscribers and organizations by calling telephone numbers they see in works of fiction.