EXHIBIT 21 PRINCIPAL SUBSIDIARIES OF SBC COMMUNICATIONS INC. AS OF DECEMBER 31, 1999 State of Conducts Name Incorporation Business Under Ameritech Corporation Delaware Same Pacific Telesis Group Nevada Same SBC International, Inc. Delaware Same Southern New England Connecticut Same Telecommunications Corporation Southwestern Bell Dually incorporated in Same Mobile Systems, Inc. Delaware and Virginia Southwestern Bell Missouri Same Telephone Company Southwestern Bell Missouri Same Yellow Pages, Inc.