American Telephone & Telegraph Co., facing increasing competition in telephone equipment sales, has turned for help to a giant retailer, Sears, Roebuck and Co.

Under an agreement announced today by the two companies, AT&T will be able to sell its equipment in most if not all 851 Sears retail stores and through Sears' 1,227 catalog sales offices.

"When you want to be dominant the best thing to do is get together with people who are dominant," Gene Stroner, a Sears' merchandise group vice president, said at a press conference.

The agreement with Sears marks the first time AT&T has sold its equipment through outside channels. At the same time, AT&T is stepping up a new program to permit its customers to buy the telephones now in their homes, which currently are leased for monthly charges.

Sears, the nation's largest retailer, already is selling a limited number of telephones produced by other manufacturers and plans to continue doing so.

Although details were unavailable, it was also clear from the announcement that the two firms also intend to work together in marketing more advanced computer-driven gear for the home.

The moves by AT&T are part of its mammoth divestiture and reorganization ordered by federal regulators and the courts.