A Zenith number was a telephone service in the United States that allowed a calling party to call the service subscriber at no charge by requesting the name Zenith and the number from a switchboard operator. The service preceded the system of toll-free telephone numbers with area code 800 in the United States. Some areas used the names "Enterprise" or "WX" number. History of Toll-Free Numbers, from bebusinessed.com
Introduced in the 1930s, a Zenith number was listed in local directories in each community from which a business desired to receive calls. In that era, direct-dial numbers were commonly published with telephone exchange names followed by digits, such as in the telephone number "PEnnsylvania 6-5000".
The letter Z appeared on many telephone dials from the early 1930s to the 1950s at the same position as the label Operator with the digit 0, indicating that the caller had to call the operator to place the call. The operator looked up the Zenith number to find the corresponding city and directory telephone number, and completed the call by charging it to the destination number.
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. For example, if a bus company had to provide a bus information number for callers in an area distant from its main office, and if a Zenith number cost $6 a month plus about $1.50 per call while an FX number cost $50 a month, then until the Zenith number regularly received at least 30 calls or more a month it would be cheaper than establishing foreign exchange 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.
As direct-dial toll-free service declined in cost, Zenith numbers nearly disappeared; telephone companies in most service areas are no longer assigning new Zenith numbers.