PRESIDENT CARTER presumably had two things in mind the other day when he declined either to condemn or to give a full report on the Soviet Union's long-established practice of intercepting telephone conversations with gear at its American installations. First, we suppose that he did not wish to condemn an activity, electronic eavesdropping, that the United States practices extensively around the world. And, second, he probably did not wish to offer information that would help other governments learn more about American intercept and counterintercept capabilities. Advances in electronics long ago rendered unenforceable the inviolability of various communications, especially when the assault is mounted from "foreign territory," such as an embassy, or from space or the high seas.

But it hardly seems enough to learn from Mr. Carter that the U.S. government has taken steps to protect national-security lines used by the White House, the Pentagon and so forth. Sen. Daniel P. Moynihan (D-N.Y.), a member of the Senate Intelligence Committee, is entirely right to point to the government's responsibility, so far largely undischarged, to warn American citizens - evidently a very large number - that their communications may be subject to systematic Soviet violation. Now that the issue has passed from the status of general suspicion to officially substantiated fact, it behooves the administration to review its counterintelligence requirements to see why the public cannot be told more about what it plainly has a right to know.

Beyond that, we have the clear impression that communications intelligence, as practiced by the National Security Agency and other agencies, is an area of government activity that has grown to gargantuan proportions over the years and that is in need of determined selective pruning. If it turned out that such pruning put the United States in a better position to demand that the Soviet Union halt its intercepts of American telephone calls, that would be a welcome bonus.