American Telephone & Telegraph Co. says it will file a petition with the U.S. government alleging that manufacturers in Japan, Taiwan and South Korea are charging unfairly low prices for small business phone gear in the United States.

" Our investigation showed a very clear picture of not just small-scale dumping, but massive dumping," J.A. Blanchard, an AT&T executive, said in a telephone interview.The alleged unfair compitition has allowed the foreign companies to increase their share of the $4 billion-a-year U.S. market to 60 percent from 40 percent over three years, AT&T said.

The companies' prices in their home markets averaged more than 75 percent higher and in one case were 170 percent higher than in the United States, AT&T said.

The equipment is for small businesses with two to 80 phone lines and consists of phones and swiches. The two major types are known as key systems and private branch exchanges, or PBXs. It is illegal under U.S. law for foreign companies to sell products in the U. S. market for less than they sell them at home, regardless of the motivation.

"One obvious possible explanation (for the dumping) is if you can sell it for twice as much in the home market, then you can grow your market share abroad," Blanchard said.

AT&T accused more than a dozen manufacturers that sell products under more than 17 brand names. Among those accused were giants such as Japan's Toshiba Corp. and Matsushita Electric Industrial Co. Ltd. Spokesmen in the United States for both of those companies declined to comment, saying they did not have enough information about the allegations. Blanchard said that while the foreign companies were increasing their market share over the past three years, the share of U.S. manufacturers fell to 35 percent from 55 percent.

"We decided we were simply not going to let that continue," Blanchard said.

AT&T said it was the first time it had leveled dumping charges against any foreign manufacturer. The petition will be filed Dec. 28 with the Commerce Department, AT&T said.

The Commerce Deparment has 20 days from the filing date to decide whether AT&T's dumping claims merit investigation.