Southwestern Bell said it bought Mast Advertising.

The purchase of Mast Advertising & Publishing for $120 million nearly doubles the number of Yellow Pages directories that Southwestern publishes. Mast, a unit of Continental Telecom of Atlanta, publishes 523 directories. Under terms of the agreement, Mast, based in Overland Park, Kan., will continue publishing directories for Contel and a number of other independent telephone companies as well as some suburban directories.