American Telephone & Telegraph Co. has announced plans to establish one of six regional marketing and service organizations for the company's new unregulated subsidiary in the Washington area.

The new organization will coordinate sales and service activities for the Chesapeake & Potomac Telephone companies in the District, Maryland, Virginia and West Virginia. The new company also will operate in the regions served by New Jersey Bell, Bell of Pennsylvania and Diamond State Telephone (Delaware).

The new organization here will be an offshoot of the new AT&T company, dubbed "Baby Bell," which as of March 1 will be involved in the offering of data processing and other unregulated communication products and services. In other words, the new company will offer every AT&T service other than basic, local telephone service.

"Baby Bell" is expected to have assets of at least $20 billion, and the locally-based organization is likely to be one of the most lucrative because of the importance of telecommunications services to this area.

The other regional offices will be located in the New York City area, Atlanta, the Chicago area, Denver and San Francisco.

C&P spokesman Web Chamberlain said more details on the organizational structure of the new company will be available about Oct. 1.

"This new organizational linkage is designed to produce quicker response time to our customers' needs, better resource utilization and make us an outfit better geared to the new competitive communications marketplace," said C&P President Robert Allen.

In connection with the announcement, C&P named the top executives of the new organization. Thomas G. Cross, vice president for business marketing for New Jersey Bell, will move here to become acting vice president for Mid-Atlantic region business services. Leroy W. Plekenpol, director of business sales at AT&T, will be vice president for Mid-Atlantic region business sales.

Thomas C. Burns, C&P's general manager for residence service and customer relations in Virginia, has been appointed the new company's sales director.