Western Union Corp. yesterday filed suit against MCI Communications Corp. and two of its subsidiaries, alleging that MCI defrauded Western Union of worldwide telex traffic.

In separate actions, Mobil Communications Corp. of America and BellSouth Corp. said yesterday that they plan to file suits against MCI over a contract dispute.

An MCI spokesman said the company will not comment until after it has reviewed the suits.

Western Union filed its suit, seeking unspecified damages, in U.S. District Court in New York against MCI, MCI International, Inc. and Western Union International Inc. -- an MCI subsidiary that is not affiliated with Western Union Corp.

The suit alleges that MCI diverted telex traffic from Western Union to the MCI network through a variety of fraudulent means. Western Union contends that MCI assigned false telex numbers to Western Union U.S. telex subscribers and made false representations to subscribers to induce them to use the numbers, thereby diverting the business to MCI.

MCI also told Western Union telex subscribers that they were incorrectly dialing Western Union numbers, and instructed them to dial in a way that diverted telex traffic to MCI, Western Union alleges.

Mobile Communications, based in Jackson, Miss., said it plans to file a suit in federal court seeking an undetermined amount of damages to force MCI to comply with an agreement between the two companies.

Under the agreement, MCI agreed to sell its cellular and paging operations to Mobile Communications for $121.5 million, Mobile said. Mobile Communications and BellSouth agreed at that time to jointly operate the cellular and paging operations acquired from MCI.