Although the telephone deregulation legislation just passed by the Senate "represents an important step forward" toward assuring competition in the telecommunications industry, it does not go far enough, the nation's top antitrust officer said yesterday.

In a hastily called press conference last night, William F. Baxter said Congress must make two key changes in the legislation before he would agree to drop the Justice Department's pending antitrust suit against American Telephone & Telegraph Co.

Several months ago, after coming under considerable pressure from the White House, Baxter said he would agree to drop the 6-year-old suit which seeks to break up AT&T into several smaller companies, so long as Congress enacted legislation to protect AT&T's competitors.

To meet this goal, Baxter suggested to Congress two lengthy technical amendments. These two amendments now must be changed, Baxter said, if he is to keep his vow.

Although the changes are technical, Baxter indicated that winning approval of them may be difficult, especially in the administration, which is split over how extensive some of the changes should be.

"I find it hard to say I am confident we will work out a position that is satisfactory," Baxter said.

The dispute is over language that would require AT&T to give its long-distance competitors the same access to its phone lines as local Bell operating companies now have.

The language, as now written, Baxter said, "would allow AT&T too wide a range of discretion" in granting access to its competitors.

Unless the language is tightened, he said, he would not consider dropping the antitrust suit, which is in trial. Baxter said he is not adverse to settling the suit, however, if an acceptable agreement can be reached.