Bell Atlantic Corp. said yesterday that its telephone customers who do not choose a long-distance telephone service in the next 18 months will be assigned to one.

The company, the parent of Chesapeake & Potomac Telephone Co., which serves the Washington region, said it will send a ballot to the majority of its millions of telephone subscribers this September, requiring them to "vote" for a primary long-distance service.

Those who do not choose will be assigned to one of the long-distance carriers -- American Telephone & Telegraph or its competitors, such as MCI Communications Corp. and GTE/Sprint -- based on the ratio of votes each company received from customers who did choose.

Customers who are assigned to a long-distance service and later want to invoke their choice will have to pay a $5 fee to be switched to a different carrier.

The smaller companies that compete with AT&T have complained bitterly to the Federal Communications Commission that the "equal access" process, mandated by the breakup of the Bell System, was not producing the anticipated competition in the long-distance market. The commission is expected to rule next week that all of the regional phone companies must use the random assignment method for customers who do not choose a service.

Equal-access service, which is being phased in throughout the nation, eliminates lengthy codes consumers must dial to reach the competitors of AT&T. The process also gives competitors the same quality of connections to local telephone networks -- needed to complete long-distance calls -- that AT&T enjoys.

Equal access is available in the District in parts of Southwest and downtown; in Virginia in parts of Springfield, Alexandria, Herndon, Columbia Pike, Vienna and Arlington; and in Maryland in parts of Bethesda, Wildwood, Hyattsville and Gaithersburg.

By Sept. 1, equal access will be available to 44 percent of all District telephone lines, 38 percent in Maryland and 50 percent in Virginia. It will be available to all subscribers in the District, Maryland, Virginia, West Virginia, Pennsylvania, Delaware and New Jersey by September 1986.

Customers are asked to choose a long-distance service as equal-access is available to their telephone exchange. Under the current system, those who do not make a choice within six months are automatically "defaulted" to AT&T.

AT&T competitors complain that method gives customers no incentive to choose a company and therefore a majority of customers are remaining assigned to AT&T.

AT&T maintains that customers who do not choose are making a choice of AT&T and that a choice should not be imposed on them.

Janis Langley, a spokeswoman for Bell Atlantic, said that under the assignment method, customers will receive a ballot three months before equal-access service is available to their line. Those who do not choose within 40 days will receive a second notice assigning them to a long-distance company. Customers still will be able to use AT&T service, but will have to dial a long code.

Customers then will have three to four months to switch to another long-distance service free of charge, Langley said. After that time, there will be a $5 fee.

Bell Atlantic's decision to assign customers was lauded by consumer activists who say the transition to competition could be jeopardized by consumer inaction. "It will be better for competition and actually better for consumers," said Samuel A. Simon, president of the Telecommunications Research and Action Center, a Washington consumer group.

Otherwise, the start of equal access service "comes and goes and people ignore it and it is not in anybody's best interest other than AT&T's," he said.

"We are pleased that Bell Atlantic has aggressively moved to discontinue some of the inequities that we saw in the process," said Don Campbell, a spokesman for MCI. "It is good news for the consumer in that it will stimulate more people to make an informed, positive and conscious decision for the long-distance service."

AT&T declined to comment on the Bell Atlantic decision. "As far as allocation, we advised the FCC some time ago that we don't favor allocation," said Howard Ruffner, a spokesman for AT&T. "We would hope whatever happens, customers experience the least possible difficulty in selecting their long-distance company, and we will work toward that."