Sometimes you can save money just by reading your telephone bill.

I learned that lesson myself this week. In reviewing my latest statement from MCI for long distance service, I found I had been charged for calls to two unfamiliar numbers, one with a Pennsylvania area code and the other with a Delaware area code. The calls, which ranged in duration from one minute to 64 minutes, had been made at odd hours of the day and night.

Altogether, the charges for the calls -- there were 11 of them -- came to $21.63 with tax.

As a precaution, and to avoid the embarrassment of complaining about charges for calls that I might actually have made, I called the two numbers. One turned out to be the boys' dormitory at Mercersburg Academy in Mercersburg, Penn. The other number was answered by a young man in Delaware who said he shared a house with several people.

I reported the incident to MCI, which credited my account for the calls, after determining that no one else had access to my home telephone. In addition, the long distance carrier said that I would be issued a new account number for future MCI calls, since it appeared that someone had cracked my secret code and was using it to make illegal calls.

"This is not a big problem -- but yes, it does happen -- to MCI and to every other long distance carrier, including AT&T," said MCI representative Gary Tobin.

With AT&T, a long distance call can be charged to the phone from which the call is made or to the credit card number of the person making the call.

The MCI system is more difficult to penetrate, Tobin said, because the user must know two combinations of numbers: First, you must know the telephone number to call to reach the MCI computer to gain access to the long distance line, and second, you must have an approved five-digit billing code to punch into the computer before your call goes through.

But the MCI system -- and others like it -- is vulnerable. Here are four ways that someone can get your code and use it to make their calls:

*Service and repair people who have access to your home may notice the MCI codes that you have stuck onto your telephone, along with your regular telephone number. They can jot those down and use them later to make calls that are charged to your account.

*Someone takes your purse or wallet containing a note with your long distance codes and begins using them to make calls.

*Teen-agers who use your MCI codes at home are careless with them, and their friends "borrow" the numbers.

*A computer hacker does random dialing until he finds the right code combinations to make calls that can be charged to you or someone else.

Tobin estimates that MCI losses from illegal long distance calls amount to less than 2 percent of the company's annual revenues of $1.5 billion. "It's not a lot of money," he said.

But for the individual who is trying to protect himself against such losses -- even when they are only nickel and dime losses -- Tobin suggests these precautions:

*Read your telephone bill so that you understand all the charges. Call the company if you have any questions.

*Read the literature provided by the carrier so you understand how your system works. MCI codes, for instance, are portable -- thus, you can use yours to make a long distance call from any push-button telephone in your general area, in most cases.

*Treat code numbers for long distance service as carefully as you treat code numbers for your electronic banking card.