On I Love Lucy, whenever Lucy or Ricky Ricardo gave out their phone number, they’d say it as, “Murray Hill 5-9975.” Even though that may look and sound like gibberish to modern phone-users, it was perfectly normal at the time. Lucy, you got some ‘splaining to do.
Phone numbers looked like this in the middle of the 20th century because of telephone exchanges. These were the hubs through which an area’s calls would be routed. Phone subscribers were given a unique five-digit number within their service area. These would be preceded by two digits—which were identified by letters—that denoted the telephone exchange you were connected to. (Before the 1950s, some cities used three letters and four numbers, while others had two letters and three numbers. The two letter, five number format—or “2L-5N”—was eventually standardized throughout the country).
via Why Did Old Phone Numbers Start With Letters? | Mental Floss.
