Donna Shor thought it was a joke. A telephone bill for $26,210.18?

It had to be a joke.

The bill from Chesapeake & Potomac Telephone Co. arrived in the mail about noon yesterday, the result of an apparent case of telephone credit card fraud, according to telephone company officials. The bill was delivered to Shor's direct-mail service at 1920 Bladensburg Rd. NE, and covered telephone calls made during a nine-day period, from April 29 to May 7.

Instead of C & P's standard slender, blue-and-white billing envelope, Shor's bill was delivered in a bulky brown envelope about five inches thick and weighing more than a pound.

According to Shor, her monthly business telephone bill usually totals about $350. She was issued a C & P credit card in March, but hadn't used it more than 100 times, she said.

Yesterday's billing included 2,000 calls, most of them to Iran and Nigeria, as well as others to El Salvador, Egypt, Colombia and Saudi Arabia. Calls to numbers in this country ranged from Houston to Hyattsville. Most of the calls were less than an hour long. They were made from pay telephone booths in the District as well as from numbers in the Maryland suburbs, Los Angeles, West Palm Beach, Providence and Baton Rouge, C & P officials said.

"I have no idea how the number was stolen," said Shor, who has been in the direct-mail business for 12 years and is president and owner of Mercury Mailers Inc. "I shield the card when I use it in public places and whisper it into the phone when I use it."

Web Chamberlin, a spokesman for C & P, said Shor's bill "is a very unusual case" and represent a well-planned and organized scheme involving several parties. "We do have some problems with credit card fraud but seldom are the bills this high. Obviously, the number is being passed around. There's no-pattern," he said.

In some instances, the credit card number was used in one city and used a few seconds later in another city hundreds of miles away.

Chamberlin said that figures detailing how much money the company loses each year through illegal use of credit card numbers were not immediately available. But he estimated that "over a period of a year, credit-card fraud in the D.C. area is certainly in excess of $100,000."

Chamberlin said the telephone company has taken steps to make sure Shor's credit card no longer registers in the company's computer system. Shor will receive a new card soon, he said."Yeah, I'll receive a new card but, I'll be afraid of it, I think," Shor said yesterday. "I've always talked a lot, but this is ridiculous."