Marketing competition for long-distance telephone service intensified yesterday as International Telephone and Telegraph Corp. announced plans to sell use of its network at counters in 21 Sears, Roebuck & Co. stores in the Washington and Miami metropolitan areas.

The ITT-Sears announcement, the first time long-distance services have been made available in retail stores here, follows the recent signing of an agreement between MCI Communications Corp. and American Express Co. under which the credit card firm will market MCI's services to its cardholders.

ITT hopes the relationship will become nationwide, reaching Sears' more than 800 stores. Washington's 10 Sears outlets and Miami's 11 stores were chosen as test sites because of the mobile nature of the two regions' residents.

"This means they depend very heavily on the telephone, in many cases spending $50 to $200 per month on personal long-distance service," said Richard Shears, director of voice services for ITT's United States Transmission Systems Inc.

USTS has about 50,000 users. About 15,000 of those are residential users compared with about 450,000 for MCI's widely advertised residential service.

The ITT service, known as City-Call, will be sold to potential residential customers at Sears key shops, which are operated under a license agreement with Cole National Corp. After signing up for the service at the key counter, consumers will receive an authorization code within a few days to begin using the ITT long-distance network, the company said.

Like customers of other long-distance competitors of American Telephone & Telegraph Co., users of the ITT service must have a push-button phone or a converter that can be attached to a rotary phone. After dialing into the ITT computer, the customer dials a six-digit billing code and then the area code and number.

An ITT representative said the company is waiving a $10 one-time registration fee and, for the first month, a $5-a-month fee for customers who sign up at the Sears stores. Also, those customers will have access to the service on a 24-hour-a-day basis. The company said that, for example, its rates for a 10-minute weekend call from Washington to San Diego would be 80 cents compared with $1.92 over AT&T's network.

"We want more people to realize it is now possible to shop around for long-distance telephone service," said John C. Reynolds, president of USTS. "We think the retail market holds a wealth of opportunity for City-Call."

Three of the Sears stores that will offer the service in this metropolitan area are in the District, three are in Maryland and four are in suburban Virginia.