YOU ARE King Khalid of Saudi Arabia and the Persian Gulf is starting to light up like a pinball machine and you have only a dime for the phone. Whom do you call?

Jimmy Carter? The United States, you know, is eager to appear a reliable patron but not well placed to provide protection and reassurance on the scene. Iran is gone as an American surrogate and Washington cannot project from thousands of miles away the power that Moscow can project from hundreds. The United States has no political access in either Tehran or Baghdad, and it cannot take any step that might hurt the hostages. You can see that Mr. Carter's principal political rival, even as he snipes at the president for events past in Iran, supports the policy of passivity that the administration terms "neutrality." Call Jimmy Carter? Nothing doing.

Leonid Brezhnev? You consider that the Soviet Union has a treaty of friendship and a strong position in Iraq, even though it seems to have turned down Iraq's appeal for arms resupply, and that Moscow has channels of influence in Iran notwithstanding the Khomeini regime's anti-communism. That gives the Kremlin a good opening to swoop in with an offer to mediate. Such an offer could douse the flames and ensure the flow of oil and thus would be hard for anyone to say no to. But -- ugly thought -- it could also put the Soviet Union politically on top of the Gulf. You know that is why the Americans are urging the Soviets to support their effort to put mediation in the hands of the United Nations, and that is why the Soviets decline. You don't call Moscow.

Kurt Waldheim? Since you are King Khalid, you already know the uses of the United Nations and you pocket your dime.

Ruhollah Khomeini? You distrust his revolution and his brand of Islam and his whole noisy boat-rocking style and you have never liked Persians. No call.

Saddam Hussein? The president of Iraq is not your favorite Arab brother: too radical, too rich, too tough, too ambitious, too close to Moscow, too independent. But he is Arab, he is next door, he is strong and, as you determine by watching nervously for four or five days, he appears to be winning. You know that any Saudi approach to him will be the acknowledgment, by the country whose acknowledgment is most meaningful, that Iraq has truly arrived. And you know that Iraq's leadership of the Arab world will hardly be aimed at the tranquillity you seek. Better an ascendant Iraq perhaps open to your influence, however, than a Soviet Union directly astride the Gulf. You put in the dime and call Saddam Hussein and tell him you stand by Iraq's side "in its pan-Arab battle and its conflict with the Persians, the enemies of the Arab nation." Arguably, it could be the most important phone call of 1980.