ONE OF THE casualties of the change in administrations is hope for a quick end to the government's antitrust case against the American Telephone & Telegraph Co. Lawyers for the company and the Carter Justice Department apparently reached a settlement. But they want the agreement to be approved by the Reagan Justice Department, and there is no Reagan official yet in place who can do that. The result is that Judge Harold Greene has postponed the trial of this mammoth case for yet another month.

It may be just as well. The case, which is aimed at breaking up Ma Bell, is more simple dispute between the government and a private company over the meaning of the antitrust laws. Its conclusion -- whether by settlement or by judicial order after a long trial -- will most likely set the framework in which the rapidly expanding communications industry will develop during the rest of the century.

Ideally, a matter of such importance should not be in the hands of lawyers and judges at all. It ought to be settled by Congress. But, as in other matters in which the issues are so complex, there is no congressional consensus on what, if anything, should be done about AT&T. If there were, Congress could have ended this costly and bitter litigation long ago.

By postponing further proceedings, Judge Greene has guaranteed that the still unselected assistant attorney general for antitrust will come under close scrutiny in the Senate. Presumably, he will have the final say on how the Reagan administration feels about the proposed settlement because Attorney General William French Smith has disqualified himself. (Mr. Smith was once a director of a Bell System company.)

It is possible that some of the details of the settlement agreement will leak out during the hearing on that nomination. Otherwise, it appears that the public will have no way of knowing what sort of future for AT&T the lawyers have in mind until a consent decree is laid before Judge Greene.

That seems to us to be unfortunate. While the case will remain open for 60 days after the decree is filed, that may not be enough time for AT&T's competitors and other interested parties to analyze its effects. It would be well if the Department of Justice -- either Mr. Carter's or Mr. Reagan's -- could figure out a way to give the public some inkling of how it proposes to settle so important a case.