This was back when Bell was “the” phone company for most of USA. The Strategic Air Command or SAC (the Air Force command tasked with managing the Air Force’s nuclear weapons) was based in Offutt Air Force Base in Omaha and needed a lot of incoming phone lines in the case of an attack. Obviously most of those lines went unused.
Mail-order companies saw potential in the infrastructure and petitioned North Western Bell and the government to allow its use. They agreed with the proviso that the businesses would be cut off in case of an attack. But at the time, using the same 800 number both for calls within a specific state and calls from one state to another was against telephone company rules. Sellers therefore had the choice of confusing their customers with two 800 numbers, one for callers within X and one for callers living in the rest of the country, or just having the one 800 number and simply using this iconic line.
Supply the iconic line…