IF COMPETITION is good for the domestic telephone service, why isn't it equally good for the international communications satellite system? Following that logic, the Reagan administration decided last month to let additional carriers get into the field. The decision is the right one, but it has consequences that go well beyond communications technology.

It appears to end the monopoly in international satellite telecommunications that has been maintained by Intelsat, a consortium of 109 countries, since the industry began two decades ago. In turn, that would end the monopoly held by Comsat, the corporation that is the American representative to Intelsat and this country's only point of access to its system. Or is it the end? Both Intelsat and Comsat lobbied vigorously against the competitive policy while the administration was making up its mind. Both continue to be against it. Today on the opposite page we publish an article by Melvin Laird, the former secretary of defense and former member of Congress, now a director of Comsat. Mr. Laird's tone suggests that he would like to see Congress take an interest in the subject.

The key issue in any diversification of the international satellite system is the disruption of the complex subsidies that it, like any monopoly, has generated. Very similar questions arose in the debate over breaking up the AT&T network here in the United States. If domestic long-distance rates no longer subsidize local service in the sparsely settled regions of the country, what will happen to rates there? Similarly, Intelsat charges more than the cost of service on the heavily used international routes but less than cost on the routes where low volumes push the cost per message extremely high. The New York-London traffic subsidizes the New York-La Paz traffic and the London-Dar es Salaam traffic.

Intelsat's competitors will go for the heavily used routes, driving prices down much closer to cost on them. The policy question -- not only for Intelsat but for the U.S. government -- will then be whether and how to maintain service to the cities where service is less profitable. Typically they will be in developing countries.

Mr. Laird raises the possibility that the Soviets might draw some of those countries into its own satellite system. That seems unlikely. Fortunately, the Soviets have never been adept at exploiting this kind of commercial opening.

The greater threat is a deterioration of communications in many parts of the world, reducing the range of the global network and impeding economic growth in countries that need it desperately. Perhaps the governments of the world's rich countries will have to replace the present hidden subsidies with open and direct support for the communications system, an unusually useful kind of foreign aid.