On the morning of last Dec. 23, presidential assistant Zbigniew Brzezinski placed a telephone call from Washington to the Soviet Mission in Geneva, Switzerland. When a Soviet functionary answered, the White House Operator asked him to summon Secretary of State Cyrus R. Vance to the telephone.

Soviet Foreign Minister Andrei A. Gromyko and the inner circle of his SALT team, in the midst of negotiations with Vance, were taken aback by the unprecedented call to an American official through a Russian telephone. They believed the call to be highly significant and noted that Vance appeared uncomfortable when he returned to the negotiating table.

It is not known how much the Russians learned from the guarded trans-Atlantic telephone conversation, but their reading of Vance's reaction was close to the mark. The secretary of state had just lost a round to Washington contenders on a difficult and persistent strategic arms issue: Soviet encoding of missile testing information, or "telemetry encryption."

This skirmish at a crucial moment last December illustrates an important fact about the lengthy negotiations for a new strategic arms limitation treaty (SALT). There were two sets of SALT II negotiations in the Carter administration: a well-publicized set of bargaining sessions with the Soviets centered on nine meetings between Vance and Gromyko, and an equally important set of internal debates in 38 meetings of the White House Special Coordination Committee and four meetings of the full National Security Council.

The skirmish also illustrates the complexity of the bargain that has been struck between the superpowers. In this case, the issue was the second "common understanding" (in effect, the second footnote) to Article XV, one of 19 articles of what is probably the most clearly defined arms control agreement in history. The rich detail on every issue provided a fertile field for the expression of suspicions and cross-purposes between the nations, to be balanced against their common interest in survival in a thermo-nuclear age.

Verification-the ability to observe and thereby police what the other power is doing-goes to the heart of the differences in history, states of mind and practices of the two nations. Secrecy has been a penchant of Russians long before the rise of communism, and it has been accentuated by a closed totalitarian system since 1977. The drive for certainty-"show me" proof-has been a strong American trait.

Technical means of verification-spy satellites, electronic tracking stations and the like-provide the underpinning of confidence that makes possible arms agreements between nations lacking in mutual trust. Thus the workings of these technological arrangements are of great political and military sensitivity.

When a ballistic missile is launched into space for testing purposes, it transmits telemetry-a stream of scientific data on its performance-back to ground stations. Intelligence agencies also are listening to check the characteristics of the other sidehs emerging weaponry.

After the 1972 SALT I agreement, the Soviets began occasionally to transmit missile testing data in code in an effort to deny the data to U.S. eavesdroppers. Although the SALT I agreement banned "deliberate concealment measures" that impede its verification, it was difficult to argue that the Soviet testing data were needed to verify the crude numerical limits of the 1972 agreement.

In the initial SALT II discussions, the Soviets proposed that they be permitted to continue "current testing practices" despite any potential impact on verification. The U.S. strongly resisted, contending that testing information would be of great importance in monitoring the more ambitious SALT II restrictions on missile capabilities.

In mid-1977 the Soviets conceded the issue of a special exemption for testing practices under the future treaty. This led to a debate in Washington over whether to press the matter further. Paul C. Warnke, then director of the Arms Control and Disarmament Agency, argued that the record for nonconcealment was strong and the matter should be left right there. However, Stanfield Turner, Central Intelligence Agency director, argued successfully that the United States should seek more explicit limits on telemetry encoding.

The problem was that the CIA and the National Security Agency insisted that U.S. negotiators not describe in detail the Soviet practices to be curbed, in fear of revealing far more than the Russians knew about the nature and effectiveness of U.S. surveillance. Washington policymakers debated how to get to the heart of the question with the Soviets, while in Geneva SALT negotiators danced around the issue.

Finally, in Geneva on Sept. 15, 1977, "the Russians said the dirty word," as a U.S. official later described it. In response to a vague U.S. statement about "data measured on board vehicles during flight tests," the Russians declared they felt free to use "various methods of transmitting telemetric information." A little later Soviet delegate Alexander N. Shuchukin, a specialist in radio electronics, spoke openly to a U.S. delegate of "encryption." The taboo subject was in the open.

The United States began to push hard for a Soviet commitment against encoding, and the Soviets displayed increasing suspicion of U.S. motives. The Russians made clear they would not ban all encoding, arguing that all the telemetric information would not be needed for the verification of SALT II provisions. The United States would not provide a list of exactly what would be needed for SALT monitoring, in fear of revealing additional intelligence secrets and in hopes of obtaining as much uncoded data as possible.

After months of jockeying, the issue came to a head last December in Geneva. On Friday afternoon, Dec. 22, Soviet negotiator Viktor Karpov and U.S. negotiator Ralph Earle, working from instructions provided by Gromyko and Vance, hammered out the language of a "common understanding" in which the Russians for the first time agreed explicitly to limit encoding.

The essence of the "common understanding" was that encoding of telemetric information would be banned whenever such an action would impede the verification of SALT. Encoding of performance data not covered by SALT agreements weould be permitted. Disputes about details would be referred to the Standing Consultative Commission established by the two sides under SALT I.

With the Karpov-Earle language in hand late dec. 22, Vance cabled it to Washington and recommended that it be accepted as the settlement of the encryption issue. President Carter had left town to spend the Christmas weekend in Plains, Ga., but Brzezinski convened what came to be known as "the midnight meeting" to consider Vance's recommendation.

Deputy Secretary of State Warren Christopher and Deputy Arms Control Director Spurgeon Keeney, whose agency heads were in Geneva, backed Vance's plea that the issue be considered settled. Keeney is described as having been eloquent on the subject. But CIA Director Turner was unhappy, arguing that a stronger, more detailed agreement would be needed for the Senate debate on ratification and to head off future misunderstandings.

"Turner was the problem," recalled one informed official. Said another high-level source: "It wasn't the technicalities so much as the political position which was at stake."

An event Dec. 21 added force to Turner's argument. As Vance and Gromyko were sitting down to negotiations in Geneva, Soviet engineers at the Tyuratam weapons test site near the Iranian border launched an SS18 rocket, headed for the Kamchatka impact range in Soviet Asia. U.S. intelligence stations were listening, but the telemetry was heavily encoded.

Word of this Soviet action, just as the issue was being negotiated at Geneva and considered in Washington, generated concern that forces within the Russian military-if not the Kremlin leadership-were sending a message of defiance.

With the State Department and the Arms Control and Disarmament Agency on one side and the CIA on the other of the midnight argument, Brzezinski took the split to Carter in Plains by telephone. In what was described as an effort at a compromise solution, Brzezinski suggested that the Karpov-Earle language be accepted, but that an amplifying statement of the U.S. viewpoint be presented by Vance to Gromyko Saturday morning in Geneva. Carter approved. The statement was drafted in the White House and dispatched to Geneva about 11:30 p.m. Washington time (5:30 a.m. Saturday, Geneva time).

Now it was Vance's turn to be unhappy. The "common understanding" had been laboriously negotiated Friday and presumably had been cabled to the Kremlin overnight for final approval. The U.S. announcement just eight days earlier of full diplomatic relations with the People's Republic of China, Moscow's arch rival, made the Kremlin posture on SALT uncertain. Now, with negotiations near completion in the Saturday morning Geneva session, Washington was adding a new element of potential controversy.

When Vance arose and read the message from the White House, he decided to take action. About 9 a.m. Geneva time (3 a.m. in Washington), Vance telephoned Brzezinski to appeal the decision. The Secretary of state did not think the newly drafted statement would be helpful in advancing the cause of clear understanding, and it might get in the way.

Carter was asleep in Plains, but as soon as he awoke Brzezinski presented Vance's objections. The president re-affirmed the decision of the previous night. Brzezinski then called Vance at the Soviet Mission, where negotiations with Gromyko were underway, to deliver Carter's answer.

Because of the high expectations of Vance and over-optimistic press reports that a SALT agreement was virtually complete, the eyes of the world were on Geneva that Saturday morning of the Christmas season.Outside the Soviet Mission an unusually large press corps waited expectantly for news of success. But inside the ornate old mansion, the negotiations did not go well.

The dynamics of what happened that Saturday in Geneva are still a matter of dispute. From the U.S. perspective, several relatively minor problems became more serious points of controversy.

Several top U.S. officials believe Moscow suddenly switched signals to prevent a final SALT agreement until the new relations between Washington and Peking could be evaluated. Other high U.S. officials believe there were simply too many loose ends to be tied together. Gromyko officially blamed "the logic of the talks" for the tortuous negotiations. Later, Soviets unofficially blamed a last-minute U.S. shift on an important issue, presumably encoding of telemetry, for failure to make greater progress.

Gromyko did not respond positively in the Geneva meeting to the telemetry statement delivered by Vance at White House instructions. In Washington, those who were dissatisfied with the Karpov-Earle "common understanding" continued to press for additional clarification. Sen. John Glenn (D-Ohio)8 briefed by Vance on the progress in Geneva, made it plain he considered the understanding insufficient and the likely seed of future Soviet-American disagreement.

Early in 1979, the United States presented to the Soviets a new version of the White House statement Vance had delivered to Gromyko. This time the Dec. 21 encoding missile test was added as an example of a troublesome event. On Feb. 14, Valentine's Day, the Soviets replied through Karpov in Geneva: There is no need for further clarification beyond the agreed statement of "common understanding."

Carter was becoming weary of the haggling over the issue, and complained to an official that the U.S. effort to "dot every i and cross every t" was reminiscent of Israeli Prime Minister Menachem Begin's maddening pursuit of semantic technicalities in the Middle East negotiations. When an exchange of letters between Carter and Brezhnev failed to clear up the telemetry issue, a March 21 meeting of cabinet-level officials at the White House adopted a new and still more tightly worded statement to be presented to the Soviets.

After approval from the Joint Chiefs of Staff and Carter, the new statement was presented by Vance to Soviet Ambassador Anatoliy F. Dobrynin at the State Department March 29. Nine days later Dobrynin gave Vance the answer: the Soviet Union accepted the U.S. statement. After months of maneuvering, the telemetry issue was settled.

At an earlier phase of the SALT II negotiations, Moscow agreed for the first time to hand over official information on the numbers of its strategic weapons, data considered highly secret by Soviet authorities. At that point Soviet negotiator Vladimir S. Semenov told his U.S. counterpart, Warnke, that "you must realize you have just repealed 400 years of Russian history."

A similar exception to the Russian tradition of secrecy is involved in the issue of coded telemetry. Because missile data secrets are on the technological frontier, the negotiations on this question have been among the most difficult of SALT II, in both capitals. CAPTION: Map, A contentious issue in the SALT II negotiations was that of telemetry, beamed back to Soviet testing stations by missiles. Missile shots from such sites as Tyuratam are monitored by the United States with spy planes, radars and listening posts, and encoding the data complicates U.S. analysis of missiles. By Dave Cook-The Washington Post; Picture, President Carter conducts SALT policy meeting. From left are David Aaron, deputy national security adviser; Paul C, Warnke, chief negotiator; Secretary of State Vance; the president; national security adviser Zbigniew Brzezinski and Reginald Bartholemew, a National Security Council staff member. White House Photo