A call for dialogue at UN.

President Reagan, speaking Monday morning to the 39th General Assembly of the United Nations, sounded an upbeat, optimistic, indeed idealistic note. He reaffirmed the United States' lasting commitment to the UN and to the peaceful resolution of disputes around the globe.

The conciliatory tone extended to the Soviet Union. The main substance of the latter part of his address centered on his willingness to improve bilateral relations between the US and the Soviet Union, and the need for arms control negotiations.

Specifically, Mr. Reagan proposed ''that the two nations embark on periodic consultations at policy level about regional problems.'' A political dialogue between the two superpowers would help avoid miscalculations and reduce the potential risk of a Soviet-US confrontation, he suggested.

His speech, presumably an early public expression of themes he will take up personally with Soviet Foreign Minister Andrei Gromyko in Washington on Friday, seized once again on the Soviet Union's proposal of last June for talks in Vienna on limiting space weaponry. Mr. Reagan claimed that ''the US was prepared to discuss a wide range of issues of concern to both sides.''

Without going as far as meeting the Soviets' precondition for such talks - namely that the US suspend all tests of space weapons - he declared himself ''willing to consider what measures of restraint both sides might take while negotiations proceed.'' This was reckoned here to be a more flexible position than he has shown previously.

He also proposed that the US-USSR negotiators ''consider the exchange of outlines of five-year military plans for weapons development and our schedule of intended procurement.'' He called for the exchange of observers at military exercises and locations. And he said that both sides should work toward an agreement by next spring whereby experts from each country visit the nuclear test sites of the other country to find effective ways to verify underground testing.

Commenting on the Reagan speech in private, one American diplomat said that Mr. Reagan did not break any new ground, particularly with regard to possible arms talks with the Soviets. He has indicated to the Soviets a ''willingness to improve relations,'' this diplomat said, but has not hinted at any substantive concessions at this time.

An ambassador from a nonaligned country remarked, ''Whether these pious intentions will in fact lead to a relaxation of tensions remains to be seen. We must take the President at his word and encourage him when he seeks talks rather than confrontation with the Soviet Union.''

In a theme that emerged early in the speech, Mr. Reagan heralded America's way to achieve success and happiness and offered this as an example to all. ''Encourage innovation and individual enterprise, reward hard work, reduce barriers to the free flow of trade and information,'' he suggested.

Then he outlined the major goals of American foreign policy aimed at ''furthering freedom and avoiding war.'' He singled out the strengthening of ties with old friends, efforts undertaken to help avoid regional conflict, and attempts made to seek arrangements with the Soviet Union aimed at reducing the level of arms.

These objectives of US policy are, in his words, ''consistent with the UN Charter.'' He insisted that ''the concern for human rights - a fundamental goal of the UN Charter - is the moral center of our foreign policy.''

Reagan listed the diplomatic efforts the US is engaged in to resolve conflicts in Southeast Asia, the Middle East, southern Africa, Central America, and the Gulf.

''We support the UN'' and ''we are proud of our role in its formation,'' he said. Then he emphasized the US commitment to UN universality and its opposition to the expulsion of any nation. Iran is expected to once again launch an attempt at getting Israel expelled from the organization this fall.

Reviewing the speech, one US diplomat said that, although it sounded flexible and reasonable, it did not in any way depart from the policies enunciated here in the past by Ambassador Jeane Kirkpatrick.