THURSDAY THE FEDERAL Communications Commission authorized what is likely to be one of the most spectacular corporate fights in history. In one corner -- the champion's -- is the American Telephone and Telegraph Company, the world's biggest utility, with annual sales of $36 billion. In the other is the contender, the International Telephone and Telegraph Corporation, 11th largest industrial corporation in the United States, with sales of $13 billion. The prize is a part of the long-distance communications business that AT&T has dominated for so long. The favorite? We don't know and, in truth, we don't care. The only thing we are concerned about is ensuring that the public does not turn out to be the loser.

The problem in this fight, as in many of the others involving Ma Bell, is that it will be hard to tell how the public interest is faring. AT&T is so big and the prices and costs of the services it sells are so intertwined that no one knows which customers, if any, are being treated unfairly. In theory, at least, the arrival of competition in some parts of its business will alter that situation. But whether that means higher or lower rates, or better or poorer service for other parts of its business, remains to be seen.

Besides being the largest private employer in the United States and the preferred investment vehicle (or so it is said) for widows and orphans, Ma Bell is a remarkably well-managed organization that has created the world's finest communications network. Those facts might lead you to wonder why any company would want to enter the ring of competition with it and why the government would permit -- actually, encourage -- that to happen. The answer is that ITT and some other companies claim they can provide certain kinds of long-distance service at rates lower than those charged by AT&T. The government, which is fostering competition throughout the economy these days, can't oppose that. Indeed, it believes that some old-fashioned competition might make Ma Bell a little less complacent about the marvels of communications it has fashioned and a little more eager to provide even better service cheaper.

What this new competition will do to your telephone rate and the quality of the service you get at home, no one knows. AT&T says if competition forces it to lower its charges to big users of long-distance service -- the market that ITT intends to serve -- it will have to raise other charges, such as those on your monthly bill, to keep its finances in good shape. Given the increasing demand for long-distance communications to transmit all manner of industrial data along with conversations, as well as the reluctance of regulatory agencies to permit rate increases, that may or may not happen.

In any event, the fight will be titanic. The development of microwave facilities and satellites has opened up a host of opportunities not only for ITT and the other companies already seeking to cash in but for every heavy user of communications, like this newspaper. Ma Bell is feeling the pressure on all sides, including a massive antitrust suit brought against it by the Department of Justice.

The question, in the long run, is not whether AT&T wins and regains its monopoly or whether its would-be competitors seize major parts of its markets, or how cheaply and efficiently big users of communications operate. The test, so far as the general public is concerned, is whether -- when the dust settles -- people will still be able to dial a set of numbers and be quite confident that the phone that rings will be in the right house. The battle now beginning between co6porate giants shouldn't have any effect on that. But each of Ma Bell's new competitors needs to be constantly aware it will disrupt that part of her empire only at its own peril.