The big casinos often had marketing offices in other large cities like Los Angeles and San Francisco. Some still do.

Where there was no local office in the city to be served, a casino could have a private line installed connecting the cities, providing a local number for gamblers to call.

From WIKIPEDIA:

With "inward WATS," subscribers are issued a toll-free telephone number, typically beginning with a designated toll-free area code. Inward WATS service was introduced by AT&T in 1967 in order to reduce time spent by operators processing toll-collect calls for businesses. The first inward WATS area code issued was 800, with 888, 877, and 866 area codes being planned and implemented in the 1990s, and 855, 844, 833 and 822 reserved for future expansion. Telephone users within a designated area may call an inward WATS telephone number without having to pay a toll charge - the recipient pays for the calls at a flat rate or other predetermined rate. Growth of inward WATS exploded in the 1980s as technology allowed companies to build business with nationwide toll-free 800 numbers which could ring at multiple call-centers.