Comment by ScottBurson

> This usage predates mobile phones

Indeed -- it dates back to the 1920s. At that time, phone numbers in the US started to be formed from a 2-letter exchange code followed by 5 (or sometimes 4) digits. For instance, "PEnnsylvania 6-5000" (dialled 736-5000) was the number of the Hotel Pennsylvania in New York City, and also the title of a 1940 hit song.

Source: https://en.m.wikipedia.org/wiki/Telephone_number