The service preceded the system of toll-free telephone numbers with area code 800 in the United States.
[2] Introduced in the 1930s, a Zenith number was listed in local directories in each community from which a business desired to receive calls.
For an organization expecting calls from a specific area but not very frequently, a Zenith number could provide savings over using foreign exchange (FX) service.
In the United States and Canada, usage of manual Zenith numbers diminished after the 1967 introduction of interstate direct-dial 800 area code InWATS toll-free service, and especially after 1982 when 800 calls no longer had to be placed via special fixed-rate trunks.
A similar service in the United Kingdom, in which callers asked operators for "Freephone (name or number)", had no direct-dial counterpart until 1985.