The president of Japan's national phone company, Nippon Telegraph and Telephone Corp. (NTT), said today the company has found defects in a variety of telecommunications equipment purchased from the United States.

Hisashi Shinto said the equipment's suppliers had been highly cooperative in working to correct the faults, which he described as minor, yet large enough to affect overall performance. The experience has not soured NTT on U.S. contracts, he said.

But Shinto said the "quality control that we seek so strictly in Japan has not been fully implemented in the U.S. . . . . I believe these are the result of sloppy work."

Shinto's remarks, made at a lunch with foreign journalists, echoed those of many business leaders and newspaper columnists here, who believe U.S. sales in Japan in a variety of sectors suffer from an often low quality of goods being offered. U.S. companies dispute this.

U.S. companies got only about $130 million of the roughly $3 billion in procurement purchases NTT made in the year ended March 31. Shinto said the main reason for their poor showing was the strong dollar rather than problems with quality.

Sales to NTT by American companies have been a point of major tension between the U.S. and Japanese governments in recent years.

Established as a monopoly government corporation three decades ago, NTT became a private corporation and lost its monopoly on April 1. The government now owns all of NTT's stock, but will begin selling part of it to the public next year.

In 1980, Tokyo and Washington signed an agreement guaranteeing foreign suppliers equal standing in the competition for NTT contracts. U.S. sales rose to $130 million by 1983 and have remained roughly steady since then.

Many of the U.S. sales have been for state-of-the-art equipment in which the United States leads Japan, such as supercomputers. Shinto declined to say on what types of equipment the problems had centered.

Shinto said foreign companies now have equal standing with Japanese firms in NTT bidding. "I have a responsibility to select the necessary equipment for us at the best price, best quality. That's the only rule," he said.

American executives here concede that Japanese companies have the lead in many product lines, though not in telecommunications equipment. They argue that a variety of formal and informal barriers keep U.S. sales here low. They also agree that the strong dollar is an important factor working against them.

John P. Stern, the American Electronics Association's senior representative in Tokyo, said it is "practically an article of faith" among Japanese business leaders that U.S. goods are inferior.

"Japanese company tests are often set up with the purpose of fulfilling that prophecy," he said.

In the meantime, Japanese companies have done a thriving business in the U.S. telecommunications market, in large part because of new opportunities opened by the Bell System breakup. In 1984, they sold $12 worth of equipment in the United States for every $1 of sales that U.S. companies made to Japan.