For the first time in a century of near-monopoly for telephone service in the United States, an American Telephone & Telegraph Co. unit is planning to hold a discount sale of home telephones.

Stating today, Chesapeake & Potomac Telephone Co. will put on sale some 300 to 400 telephone sets at selected Phone Center Stores in the Washington region. Prices will be reduced 25 percent below normal retail prices for AT&T's sale items.

AT&T subsidiaries around the nation began entering the new world of retail business several years ago, after conceding reluctantly that federal government policies mandating competition in the sale of telephone equipment were not going to be changed.

By the end of this year, regional AT&T subsidiaries such as C&P will have opened 1,900 retail telephone stores, including more than a dozen in the Washington metropolitan area, according to Susan Ames, recently named by AT&T to a new job as Bell System "home consultant"

In an interview at C&P's retail store on 14th Street NW., at Columbia Road, Ames said sales of individual "design line" telephone sets in AT&T RETAIL STORES LAST YEAR ROSE TO MORE THAN 800,000 FROM 400,000 IN 1977.

TRADE REPORTS HAVE INDICATED THAT NON-BELL SYSTEM TELEPHONE EQUIPMENT RETAILERS ARE SOMEWHAT MORE PESSIMISTIC ABOUT THEIR FUTURE MUSCLE AND GOT INTO THE BUSINESS.

ACCORDING TO C&P officials ,some 80 percent of all new D.C. residential telephone decisions - style, color, length of cord - now are made at the company's retail outlets. Up until several years ago, all such business was transacted over the telephones in calls to a C&P business office.

Customers who buy their telephones from C&P or another AT&T unit actually buy only the decorative housing; the telephone company owns the working parts. However, customers may purchase entire telephone sets elsewhere and hook them up to the telephone network in their homes, using jacks.Industry insiders say it is only a matter of time before the telephone company begins to sell the whole set.

Currently, if a Bell System set develops working problems, the company sends repair persons to do the job. If you own a non-Bell System set, repairs are your responsibility. In the future, AT&T probably will follow the same pattern and sell complete sets. Then, if a customer wants to assure repairs, the company probably will offer a service policy much like department stores now sell withnew laundry equipment.

By purchasing a set today at a retail store, customers simply take the telephone home and plug it in-saving installation charges. C&P officials estimate that more than 40 percent of D.C. housing units now have jacks that permit immediate connections.

As testimony to the new era of competition, not all sets available from C&P are manufactured by Western Electric, AT&T's equipment subsidiary. Customers may select "antique gold," "Mediterranean" or "early American" models of Merican Telecommunications Corp., a Mickey Mouse phone marketed by ATC in conjunction with Walt Disney Productions or a "coquette" by Fujikura Plastics Co. Ltd.

Prices at the 14th Street store range from $65 for an AT&T "sculptura" to $130 for the "antique gold" set. C&P sets that go on sale tomorrow are from an inventory of mostly discontinued lines. CAPTION: Picture, Karen Harris, left, C&P phone center manager, discusses new bridal phone line with company consultant Susan Ames. By Tom Allen - The Washington Post