Just as U.S. telephone customers are being bombarded by appeals from competing long-distance companies, hundreds of thousands of households and businesses may have been connected to the wrong companies.

"We are noticing how rampant a problem it is," said Susan Katz, of the Washington TeleConsumer Hotline, which provides information on phone companies nationally.

"People are getting a company that they don't have any recollection of ever signing up with, or they don't get the company at all," she said.

Such errors were admitted yesterday by AT&T, the nation's largest long-distance company. AT&T disclosed that it has pulled the plug on the computer system that keeps records on business customers that choose to stay with the company's long-distance service.

According to the Justice Department, which is investigating allegations that AT&T and other companies are misdirecting customer requests, "hundreds of thousands" of customers have been erroneously assigned by the AT&T computers alone.

AT&T voluntarily halted its computer operation after an independent accounting firm identified inadequacies in the computer system. The action means that business customers cannot select long-distance service by calling AT&T; instead, they must call the local Bell operating companies.

Residential customers are not immediately affected; however, AT&T said it is examining the computer that processes those requests for similar problems.

The Justice Department said that it found no evidence that AT&T willfully submitted orders for customers who had not selected their services, but said that even after initial errors were discovered last spring, "AT&T failed to exercise sufficient care to insure that its systems for processing orders for business customers were reasonably accurate."

AT&T said yesterday that it disagreed there had been any "systematic" problems with its computer system and estimated the number of misassigned orders at around 4,000.

Nonetheless, many telephone customers have their own tales of long-distance confusion.

For example, when Beverly Guberman, secretary-receptionist for Media Reactions Inc., a Reston advertising agency, tried to sign up for GTE Sprint several months ago, she ended up with SBS Skyline, a competing long-distance service.

"Sprint said they would start the paperwork," said Guberman.

"I got a letter from SBS Skyline that said service was effective immediately.

"They just took all of our lines, but I never authorized them," she said.

Chesapeake & Potomac Telephone Co. of Virginia directed Guberman to call the long-distance companies to straighten out the problem.

Now, Guberman has Sprint, along with an SBS bill of $275.15 for a service she had discontinued three months before.

"I think this is a pain in the neck and an unnecessary nuisance," she said.

There are numerous ways the sign-up procedure can misfire, especially since there are no industry standards for double-checking orders and ensuring accuracy, say industry officials and consumer advocates.

But making the process work is the crux of ensuring long-distance competition in the marketplace.

Known as "equal access," the process was mandated by the breakup of the Bell System and eliminates lengthy codes customers previously had to dial to use AT&T's competitors.

By next September, 46 million customers, with more than 100 million telephone lines, will have a choice of long-distance service without having to dial the lengthy codes.

William Garrett, AT&T Communications vice president of carrier selection, said the entire equal-access process was open to problems.

"I would hope that the other companies involved in . . . selection will go through a similar quality-assurance process," he said.

"This is a unique and unprecedented undertaking, both in size and complexity, with none of the participants controlling the quality of all the data required from end to end," Garrett said.

The problems with the AT&T computer came to the attention of the Justice Department last spring, when AT&T volunteered that it had had problems with its computer software.

The Justice Department had also received complaints at that time from long-distance companies, among them MCI Communications Corp.

MCI last spring charged AT&T with "actively interfering" with the process of equal access by submitting "thousands of bogus AT&T presubscription orders from all regions of the country."

Yesterday, AT&T, in conjunction with the Justice Department report, agreed that it will not resume operation of its computer system, or process any data, until AT&T and an independent consultant conclude the system is able to process orders with sufficient accuracy.

AT&T also said it will make payments to those business customers misassigned in the territories served by Bell companies Nynex, BellSouth and Southwestern Bell.

If AT&T fails to live up to its commitments, it could be liable for up to $1 million for each day of noncompliance, the agency said.

Long-distance company officials agree that problems with equal access are reaching a feverish pitch.

Jack Schiff, director of SBS Skyline marketing operations, said "I would say the phone rings off the hook."

Long-distance companies are concerned enough that they are conducting their own studies on the order-taking process.

"We are dealing with a very complicated process and with large volumes of customers," said Richard Petty, a spokesman for GTE Sprint.

"We are doing some studies to be sure we are not losing any orders, to be sure we are not delaying any orders and that we are getting all our orders from the Bell operating companies," he said.

The long-distance phone companies are blaming the local phone companies that provide the customer with service, but consumer advocates like Katz say that long-distance firms sometimes hold onto orders for several weeks, which delays service to the customer.

"Most of the Bell operating companies have not developed systems to make sure they don't lose orders or get them wrong, and we've had similar problems that delay orders," said one official.

"Like we send orders to the Bell operating companies, and they reject them for things we couldn't possibly know -- like if the customer is a business or a residence," the official said.

"There is no format for standardized information flow," said Schiff.

"We've had a distressing number of cases where people for a day or two get blocked because we haven't been notified by the local phone company" service should be started, he said.

In August, MCI sent the Justice Department a letter about the process.

"We said we've been having problems with the Bell operating companies with processing orders in a timely fashion, connecting our customers, notifying customers they had been connected, and of notifying us of conflicts between orders, said spokesman John Houser.

"Just itty bitty things like that with all the Bell operating companies," he said.

But MCI claims the problems are continuing. "We have our own special problem with AT&T," said MCI's Houser.

"AT&T is calling up a customer after we've got them signed up and sending them information anyway," he said.

"The next thing we know they are signed up, we are not notified that the customer has been switched by the Bell company and the customer gets mad at us when he gets his first AT&T bill."

"It's a complicated process; there have been mistakes on both sides," said Herb Linnen, an AT&T spokesman.

"There is no effort from our part to steal customers, they have to choose and are choosing, this is a very competitive situation," he said.

Complicating the process further is a change in how customers sign up for service.

Last summer, the Federal Communications Commission ordered the use of a balloting procedure to increase the number of customers choosing a long-distance service. Before balloting, a customer would automatically stay with AT&T if he made no choice.

Now, customers who don't fill out ballots will be assigned to a long-distance service by the local operating company.

Up until this summer, long-distance service was arranged with the long-distance company, which then notified the local company.

Now, customers are required to fill out a ballot, which goes to the local phone company.

The local company then notifies the long-distance company and turns on service. The shift itself has led to problems.

"It's been a real challenge for us to adjust to it after a number of changes in the process," said Sprint's Petty.

Both long-distance and local company officials say a standardized format is being developed, but it remains to be seen if all companies will follow the voluntary standards.