If this is the season to be jolly, it's also the time to be careful, especially when calling from a public telephone.

Industry experts warn that the holiday travel period is a prime time for thieves who prey on unsuspecting phone-card holders.The crooks often resort to ingenious methods when stealing card codes, going so far as using binoculars and opera glasses to spy on callers in airports, train stations and bus terminals.

There's the case of the Pittsburgh woman who used her calling card at the Philadelphia train station one evening last month. When the woman tried to use her code the next morning, the phone company reported that more than 100 long-distance calls had been billed to her number - in less than a day.

"The main thing is for people to treat the calling card just like they do their regular bank credit card," says Rami Abuhamdeh, executive director of the Communications Fraud Control Association, based in McLean, Va. "They have to be careful with where they use it, how they use it and who's around them."

Theft of the cards, along with other forms of telecommunications fraud, produces annual industry losses estimated at $500 million.

A telephone calling card allows calls to be made from anywhere by punching in, for instance, a 14-digit private code that consists of the cardholder's area code, home telephone number and a four-digit suffix. This means that even if a thief knows a home telephone number, he has only a 1-in-10,000 chance of correctly guessing the suffix.

Experts note that spying is not the only way to steal card numbers. Even more common, they say, are scam artists who telephone people at home and trick them into giving out their secret codes.

For example, someone claiming to be with the security department of a phone company might call a home and explain there have been abuses with the resident's calling card. Knowing the first 10 numbers are the person's phone number, the scam artist may ask, "Is your card number 412-345-6789-1234?" The unsuspecting consumer may in turn respond, "No, my number is 412-345-6789-1111," thereby giving away the secret code.

Industry officials say telephone companies never call someone and ask for a code number. "If the phone company calls you up, they know your calling card," Abuhamdeh says.

The best advice for guarding against phone fraud is quite simple: Be cautious.

"There's no reason why you shouldn't be a little guarded when punching in your card number," says BurkeStinson, a spokesman for AT&T.

Phone companies and phone security experts suggest a variety of ways consumers can guard against code theft:

- Before you use a public phone, look around. If someone is watching you, hang up and use another phone.

- When using a public phone, particularly in a busy place, try to shield the numbers with your body. Thieves with binoculars can spot you punching in your code easily, and even a seemingly innocent passer-by may be able to pick off your code.

- If you're giving an operator your code number, don't speak loudly enough for bystanders to overhear.

- Don't use your calling card as a form of identification. Authorities talk about one case in which a store owner who took checks specifically asked customers for a calling card. The store owner was compiling code numbers and selling them.

- If an unsolicited caller claims you've won some sort of contest and asks for "verification," don't give out your calling-card number or any credit-card number.

In major cities, code numbers commonly are offered for sale. It is not uncommon in some places, authorities say, for people using pay phones to be approached by thieves offering unlimited use of a card number for less than $5.

"You can find within 30 days a six-figure phone bill can come rolling in," Stinson says. While customers rarely are charged for calls billed to their stolen numbers, phone companies stress the customer is responsible for safeguarding the secret code.

"We would try to do what's right and responsible," says Stinson. "But in the strict sense, you are responsible."