The story
From 1, used to dial long-distance numbers and 800, which was, for a long time, the only North American toll-free area code.
Etymology from Wiktionary, CC BY-SA 4.0.
1-800
one·eight·hun·dredToday it means: A toll-free number or a long-distance telephone numbers; using any of North American toll-free area codes 800, 888, 877, 866, 855, 844 or 833. Full definition →