The president of American Telephone & Telegraph Corp. has testified that AT&T's New York Telephone subsidiary never tried to take steps to put MCI Communications Corp. out of business and that it never attempted to impede MCI's efforts to set up a rival long-distance network.

William M. Ellinghaus, who headed the New York Telephone Co. from 1970 until 1976, admitted last Friday that the phone company made some mistakes in providing hook-ups for MCI's customers.

However, the company investigated every one of the complaints filed with it by MCI in 1974 and discovered that "the majority of the [service problems] . . . were the fault of MCI" rather than AT&T, he said. "I must hasten to add that we were not guiltless or faultless but the majority were the result of failure on the part of MCI."

Ellinghaus was the final defense witness in the $2.7 billion antitrust suit MCI filed against AT&T charging that the telephone giant tried to choke off MCI's attempts to enter the long-distance telephone business during 1971 and 1974.

MCI immediately began its rebutal testimony by calling Bernard Strassburg, who from 1963 until 1973 headed the Federal Communications Commission bureau that oversees both MCI and AT&T. The trial is scheduled to resume next week.

Strassburg said that the 1971 FCC decision permitting competition in providing private long-distance services meant that companies such as MCI should be afforded all forms of specialized services by AT&T.

One of MCI's prime arguments in the four-month-old trial is that AT&T denied the company access to certain services it contends the FCC ordered the telephone company to provide in its 1971 decision.

AT&T Chairman John deButts testified last month that the original 1971 ruling required the company to do no more than provide MCI and other companies "local loops" to connect MCI customers with long-distance telephone service.

Strassburg, now a private consultant, testified that the commission meant AT&T should provide "practically all services" to its competitors.

Robert Hanely, the chief MCI lawyer, asked Strassberg why the decision didn't spell out what services AT&T was required to provide. Strassberg replied that the FCC would have "had to tick off every class and subclass of service" and that any such listing would have been restrictive because the commission "anticipated an evolving spectrum of services" in the wake of its decision.

Ellinghaus said that MCI officials wrote New York Telephone twice in early 1974 and again in October 1974 to complain about scheduling and installation problems. Each time the telephone company investigated and found most of the problems were due to MCI's failures rather than the telephone company's.

The final letter from MCI Chief William McGowan, charged that the telephone company was either engaged in "deliberate plotting" or that the company's ability to function had collapsed.

Ellinghaus said that after seeing the results of New York Telephone's investigation of earlier MCI complaints, "I simply couldn't believe they were complaining about New York Telephone . . . I felt there must be something else involved here," alluding to the antitrust suit which had been filed several months earlier -- the suit that finally went to trial last February.