The Bell System's new Middle Atlantic operating company, which will oversee activities of the four C&P Telephone Cos. and three other Bell companies, has chosen Courthouse Plaza in Arlington County for its headquarters.

Robert E. Allen, board chairman of C&P and president-designate of the new organization, made the announcement yesterday.

By the end of the year, 1,200 employes of the as-yet-unnamed regional company will be working at the new headquarters, Allen said.

Virginia Gov. Charles E. Robb was present at the announcement yesterday to express his pleasure with Bell's move to Virginia as Allen began detailing some of the activities the Bell service company will undertake when, pending judicial approval, it comes to life Jan. 1, 1984.

Because of the Justice Department's agreement with American Telephone and Telegraph Co. in 1982, AT&T is required to divest itself of its operating companies, including C&P. As a result, the 22 Bell operating companies have been realigned into seven operating groups.

The Middle Atlantic group consists of the four C&P companies--D.C., Maryland, Virginia and West Virginia--plus New Jersey Bell, Bell of Pennsylvania and Diamond State Telephone, the Bell company of Delaware.

The company will have more than 75,000 employes.

Northern Virginia was chosen as the regional headquarters in part because of the state's aggressive efforts to have Bell locate there, Allen said, and because of C&P's proximity to one of its largest customers, the federal government. Staff in the other Bell operating companies will be "streamlined and consolidated" at the Northern Virginia headquarters, leading one Bell source to estimate that staff will be reduced by 25 percent or more.

Allen said that he hopes the changeover to the new company would be "transparent" to consumers, with bills and inquiries still being handled by the local operating companies. The regional company, however, will be in the position to sell customers equipment such as telephones, and enter into joint ventures with information providers such as newspapers.

The new company will be incorporated, named and operating on a trial basis by Oct. 1.