MCI Communications Corp., already giving the Bell System headaches with its $2.7 billion antitrust suit, is now preparing to open a major advertising blitz designed to garner residential customers and others to its telephone system.

That advertising campaign now targeted for three trial markets -- Kansas City, Denver and Cincinnati -- would be the company's first foray into conventional broadcast and print advertising and the first major effeort by any of Bell's competitors to expand its market from a select business clientele to a broad consumer audience.

"There has never been a broad ad campaign directed at all the people who use Bell." said Gary Tobin, MCI's senior public relations manager "We don't know of anybody who has ever attacked Bell quite like this."

The advertising blitz, which may begin in early March in at least one of the three markets, will be an effort to garner more of the $18 billion telephone market for MCI, the Washington-based telecommunications concern.

Although MCI has used a limited advertising campaign primarily in trade publications to attempt to make inroads into the telephone market cornered by American Telegraph & Telephone Co., this new campaign will be directed at informing a broader consumer audience about the company's services.

MCI offers four basic telephone services -- Execunet, private line, network and residential. But the bulk of its customers in its 70 city network are businesses.

MCI's residential marketing division is slowly developing a network that would link home phones to other private-line customers.

International Telephone & Telegraph's subsidiary, U.S. Transmission Systems Inc. which offers similar services, also has never before undertaken a broad based consumer advertising campaign.

A spokesman for the system said yesterday that ITT officials view the MCI program as a good one for AT&T competitors. "Telling people about the generic concept of long distance for less is certainly worthy," said ITT spokesman Stephen Yesenosky.

MCI's landmark antitrust suit against AT&T, currently being tried before a jury in a Chicago federal court, is based on the company's allegations that the Bell System blocked MCI from obtaining necessary interconnections in developing its long distance business in the early 1970s.

Since the Federal Communications Commission opened the intercity telephone field to competition in 1971, MCI and others have been challenging AT&T, primarily for bulk telephone users, such as large businesses.

Tobin said,however, that MCI's advertising has been limited to professional publications and that the new advertising campaign, if successful in the three trial markets, could dramataically expand the company's lines of business.

Last year, MCI reported profits of $7.1 million, another step in a change of fortune for the company. In 1975, for instance, MCI lost a record $38.7 million.

The advertising campaign, however is designed to expand the company's market. This program is designed to attract residential business, a field the MCI has barely entered.

For its advertising program, MCI has hired Ally & Gargano, a New York advertising agency whose clients have included Federal Express Corp.; Saab-Scania the Swedish automaker; and Pan American World Airways Inc.