A federal judge has ordered Bell Atlantic Corp. to sell cellular phone and paging businesses it operates outside its northeastern region because those activities violate the antitrust decree that broke up the Bell System.

Bell Atlantic of Philadelphia, the parent of Chesapeake & Potomac Telephone Co., had maintained it did not need special permission to provide communications services outside its region through A Beeper Co., the national radio paging company based in Altanta that it owns.

A Beeper resells cellular mobile telephone service and acts as an agent for paging services. Cellular telephone service uses a network of radio transmitters to transmit calls to mobile telephones.

U.S. District Judge Harold H. Greene, who presided over the Bell System breakup, said in an order issued Feb. 14 that "activities which violate the decree are not to be tolerated . . . must cease forthwith and . . . may result in the imposition of sanctions." Justice Department officials would not say what those sanctions might be.

The decree that broke up the Bell System does not allow regional phone companies to offer local telephone services outside their local areas without permission, Greene's order said. "Bell Atlantic will therefore be ordered to cease its offending activities on or before April 13, 1986, or face sanctions above and beyond those which may be imposed for its activities prior to the date of this order," he said.

Greene made it clear in a previous order that regional phone companies are to move more slowly in the area of telecommunications services. Although he has approved more than 50 waivers to enter businesses from real estate to foreign consulting, the judge has admonished the companies for their "rush to diversify" and for their "relative lack of interest in basic telephone service itself."

Bell Atlantic Chairman Thomas E. Bolger said yesterday, "We entered the paging business in the good-faith belief that it was an activity permitted by the decree inside and outside of Bell Atlantic's territory." Bolger said the acquisition of a piece of A Beeper in 1984 first was carefully reviewed with the Justice Department, which "did not indicate this activity would be a violation of the decree.

"We intend to ask the judge to reconsider his latest order." However, he said Bell Atlantic has already made plans to divest the cellular part of the business.

As part of Greene's order, Southwestern Bell Telephone Co., based in St. Louis, was told to discontinue its cellular resale operations by April 17. Southwestern Bell said it had no problem with Greene's order.

Pacific Telesis, which is based in San Fransisco, was given until April 17 to dispose of its cellular resale business. Pacific Telesis already had decided to sell those assets in an earlier business decision.

But the regional company is awaiting a decision by Greene on whether it can acquire Communications Industries, a national paging and cellular service company with 1984 sales of $98 million.

The Justice Department has recommended that Greene approve the waiver. But in hearings a week ago, Greene said that, if all seven companies could acquire cellular business outside their service area, they eventually might form a nationwide monopoly.