Why do movies use 555 numbers?

The phone companies began encouraging the producers of television shows and movies to use the 555 prefix for fictional telephone numbers, roughly during the 1960s. One of the earliest uses of a 555 number can be seen in Panic in Year Zero! (1962), with 555-2106.
In older television shows from the 1950s or 1960s, "Klondike 5" or "Klamath 5" was used, as at the time the telephone exchanges used letters and numbers in phone numbers. More recent works set in this period typically use this convention as well. For example, in Back to the Future Dr. Emmett Brown's 1955 phone number is "Klondike 5-4385" while the 1985 Jennifer Parker character writes the number 555-4823 on the clock tower flier telling Marty to call her at her grandmother's.
In 2011, the fictional Netflix Relief Fund satirized the company's price increase of that year and used the fictional number 1-555-368-7147...
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555 Wikipedia
TV Tropes FiveFiveFive
A compilation of 555 numbers