He was sure the telephone bill was wrong. As he looked through the itemized list of long distance calls, he realized he was being billed for two calls to unfamiliar numbers in towns where he did not know anyone.

But he was not sure it was worth complaining. How do you prove you did not make a call when the phone company records show you did?

The Chesapeake & Potomac Telephone Co. says it receives complaints about an estimated 82,000 toll charges a year that customers in the Washington area claim they did not make. It acknowledges that in 85 percent of those cases, the customers are right, the company wrong, and the charges are removed from the customer's bills.

But C&P says it rarely gets stuck with the bill. In 92 percent of the cases in which an error has been made, a spokesman said, the company eventually finds the person who should have been billed and collects its money.

But the money for those calls, about $210,000 a year, would have been taken from the wrong people if the incorrectly billed customers had not spotted the errors and fought to have them fixed.

There is no way of knowing how many billing errors go undetected.

"The customer is responsible for looking at his bill and letting us know if there is a mistake," said Web Chamberlin, a spokesman for C&P Telephone Co.

The service representative has the authority to strike the inaccurate charge immediately -- but rarely does. Instead, he or she agrees to investigate.

The customer has the right to withhold payment of the disputed amount while the investigation is under way.

As part of the probe, Chamberlin said, "We would check our records to see if the customer had called the (disputed) number before. We would check to see if anyone at the other end of the call knew who made it."

In a typical case, the company's service representative dials the disputed number and tells whoever answers that there is some question about a call made to that number last month.

The service representative may ask, "Who called you?" and may say something like, "Somebody has to pay for the call. Do you know who should be billed?"

Implicit in the questioning is the idea that the person who received the disputed call may be charged if he or she does not help the company find the responsible party.

It isn't clear that the company could charge the person who received the call, but its tactics almost always work.

Company officials say that about 12,000 of the 82,000 complaining customers each year are found to be responsible for the calls, either because they or someone else in the household -- teen-agers, houseguests or spouses -- actually made the calls.

Of the 70,000 erroneous billings the company investigates, only about 5,700 are never traced to the proper caller, Chamberlin said. That amounts to about 8 percent of the known errors and costs C&P in the neighborhood of $17,000 in charges that eventually are just written off.

Chamberlin sought to downplay the number of billing errors, describing them as "just a small percentage" of the 192 million toll calls originating here each year.

But Brian Lederer, who represents customers in cases before the D.C. Public Service Commission, said: "Most people have this problem from time to time."

Billing mistakes occur most often during what the company calls operator-assisted calls, those in which an operator helps place a call or takes billing information from the caller. The operator may misunderstand the number of the telephone or credit card account to be billed, or the person placing the call may give the operator the wrong billing number.

Direct-dialed calls are handled by automatic billing machines that can break down and charge the wrong number for a call. "The machines have a high degree of accuracy," Chamberlin said, "but they can malfunction."

Some of the errors result from outright fraud. Chamberlin said those include people who deliberately give the wrong number to the operator.

Telephone company practices sometimes are responsible for complaints from customers insisting they did not make a call for which they were billed.

For instance, a woman in Texas recently opened her monthly statement and found a charge for a toll call to Perrine, Fla.

"But I don't know anybody in Perrine," she told the phone company. The service representative called the number in question and learned that Perrine is part of metropolitan Miami. When the customer was informed of the Miami connection, she immediately realized she had called someone there and agreed to pay the bill.

Customers who complain to the phone company but can not get satisfaction there can appeal to their Public Service Commission, which can act as a mediator in a bill dispute.

Tom Hatem, chairman of the Maryland PSC, said the company is not likely to press its demands for payment that far.

"They don't want billing problems or service troubles to get mixed up with their requests for rate increases," Hatem said.

In the District, telephone users have the added protection of the recently enacted consumers' bill of rights. Lederer, the consumer representative, said the spirit of the new rights' bill places the burden of proof on the company.

"They have to prove the customer made the call, rather than the customer having to prove he did not make it," Lederer explained.

But Chamberlin noted the telephone company still holds the ultimate weapon in disputes with customers:

"If you don't pay, we can stop service."