The telephone call that came to the comfortable, if heavily defended, suburban home of rightist politician Lionel Sisniega Otero at 8:30 a.m. last Tuesday was cryptic: "Today is the day. Join us."

Sisniega needed no explanation. Defeated in his bid for the vice presidency only two weeks earlier through what he considered government electoral fraud, he knew the telephone call was confirmation that the coup d'etat he had plotted in revenge with young military officers, some from his lieutenant son's "Kaidil" commando unit, was finally on.

For a few fleeting hours after the call, as he joined the young officers and their troops surrounding the presidential palace in downtown Guatemala City, and later when he went to the Army-captured TGW radio station to make the first announcement of the coup's success, it looked like Sisniega's moment in history had finally arrived.

Instead it was retired general Efrain Rios Montt, 55, the coup makers' chosen figurehead, who arrived on the scene and who, within a matter of hours, had wrested away Sisniega's visions of power almost as neatly as--by Sisniega's account--the ousted government of Gen. Romeo Lucas Garcia had stolen the election from his ticket.

Today, five days after the coup against the unpopular and feared Lucas government, there remains a good deal of confusion about the future policies and intent of the three-member military junta led by Rios Montt. But what is clear is that he almost singlehandedly has turned Guatemalan politics on its head, giving this turbulent Central American land besieged by guerrilla war and near bankruptcy a government different from that sought by the young officers who put him in power.

The lieutenants, captains, majors and colonels who planned the coup wanted two things: first, an end to the "fraud and corruption" of Gen. Lucas Garcia and his cronies which they considered an outrage to the honor of the Army and nation; and second, free elections within 60 days to clear Guatemala's sullied international reputation and make it possible for Washington to extend military aid that had been suspended because of human rights abuses.

Talks with sources close to the young officers' movement, diplomats, local politicians and a few of the key plotters, most of whom do not want their names used, have established that the decision to overthrow the government was taken by no more than 10 outraged officers only days after the March 7 national elections to pick a successor to Lucas Garcia.

Those elections, now clouded in allegations of fraud, resulted in a declaration of victory for Lucas Garcia's handpicked military successor, former defense minister Gen. Angel Anibal Guevara, over Sandoval and two other conservative candidates.

What angered the young officers whose ranks grew eventually to more than 200, was the election, which many of them said they had been forced to manipulate under orders from their pro-Lucas commanders, as well as the use of tear gas and police repression at a protest demonstration two days after the vote.

That demonstration was led by Mario Sandoval Alarcon , the extreme-rightist head of the National Liberation Movement, and the other two losing presidential candidates. All three were detained briefly for leading the demonstration to present charges of electoral fraud to the president.

Because the plotters, like many of their officer class, had fathers who belonged to the anticommunist, critics say neo-fascist, National Liberation Movement, there was widespread anger at the spectacle of the government's security forces quelling the demonstration.

Sisniega, in an interview, said he was asked to act as a liaison to help the officers line up civilian support for the coup and in sounding out various generals whom they could not approach about their readiness to back the plot.

Sisniega says that in the course of the eight days during which the coup was planned, he talked to five generals who were "in the mood," including Rios Montt.

On the basis of the responses passed to the officers by Sisniega, the military plotters, he said, decided privately on Rios Montt, whom most of them knew as a former garrison commander or as their professor when he headed the Guatemalan military academy.

Rios Montt's strength lay in his reputation for honesty and the fact that he felt he had been cheated out of being elected president in 1974. He spent four years in imposed exile as a military attache to Madrid, and four other years preaching for the evangelical Church of the Christian Word.

When Sisniega was alerted Tuesday that the coup was on, Rios Montt was teaching at his school, unaware that the coup was taking place.

Sisniega and other supporters rushed to the heart of the city after being alerted of the coup. They met on side streets, with their right shirt sleeves rolled up and their left down, as an identifying sign for the soldiers who, dressed in the same manner, were already converging to besiege Lucas Garcia in the presidential palace.

Army announcers who had taken over the radio station at the same time summoned both Sisniega and Rios Montt to the station. Sisniega, getting to the station first, shortly before noon, wrote and broadcast the first announcement of the coup's success.

As Sisneiga spoke on the radio, word reached Rios Montt that he was being called to head the coup. He first called together the elders of his church, who convinced him it was a sign of God and that he should go to join the coup makers. He was driven there by one of his fellow churchmen in a Volkswagen.

Rios Montt's first act was to order Sisniega from the offices of the credit union and to ban any other politicians from hobnobbing with the coup makers. After experimenting with different military and civilian-military juntas, Rios Montt settled on a senior, three-member military junta, which included Brig. Gen. Luis Maldonado Shaad and Col. Francisco Luis Gordillo, the head of the capital's general staff headquarters.

The young coup makers, who had not thought much beyond the military tactics of getting rid of the Lucas Garcia government, were quickly reminded by Rios Montt of the need to maintain the hierarchical system of the military and relegated to the role of "advisers to the junta," which seems for the time being to have limited their power.

There has been no further mention of elections. Rios Montt not only publicly denounced "cheap politicians and the same old faces," but also banned political party activity in his first government communique.

Some of the disgruntled young officers and many National Liberation Movement politicians have charged that Rios Montt has made no call to elections because he is authoritarian and power hungry.

The more charitable, including many moderate politicians such as Vinicio Cerezo, the head of the centrist Christian Democratic Party, are not so displeased at Rios Montt' silence on elections. They say privately that he seems to be showing realism about the fact that any quick elections would bring the National Liberation Movement to power, because it is at once the best organized and richest party, supported as it is by many of the rich businessmen and coffee and banana plantation owners who have long dominated, with the military, political life in Guatemala.

Politicians like Cerezo, and others of the semi-underground Social Democratic Movement, have emerged as defenders of a moratorium on elections for six months to a year, to give their parties time to organize a serious opposition to the National Liberation Movement.

"The dilemma of Guatemala today," said one diplomat unhappily, "is that for the moment the choice may be between having a Mussolini as president or a Hitler."

The big question which remains unanswered to date is whether Gen. Rios Montt will step out of power after accomplishing his stated goals of purging the country of the corrupt and repressive officials of the Lucas government. Certainly Sisniega fears he will not.

"To me there was an error," he told a visitor the other day. "We politicians have been forced to go back to our parties and allow the military junta to run the government. It seems they don't have enough sense about what is going on. They think the problem is military rather than political, and that has always been the problem of our rulers."