Wiktionary

1. 1-800(Noun)

   A toll-free number or a long-distance telephone numbers; using any of North American toll-free area codes 800, 888, 877, 866, or 855.

   866-555-1234... that's a 1-800 number, isn't it?

Origin: 1, used to dial long-distance numbers + 800, which was, for a long time, the only North American toll-free area code