A credit card number is found on the back of the card next to the security code number. It is the number that comes before the three-digit security code number on Visa, MasterCard, Discover, JCB and Diners Club cards. For the American Express card, the main card number is located on the front of the card with the four-digit security number above it.
The first few digits of a card number define the type of card. In a standard 16-digit credit card, the issuer of the card determines the first 15 digits. The last digit, also called the check digit, is computed mathematically on the basis of the other digits. The check digit is worked out to correct input errors when credit card numbers are being supplied manually. The protection makes it difficult for hackers or other persons to generate random card numbers accurately.
It is inadvisable to provide credit card numbers to individuals or parties over the telephone unless the card holder initiated the call. Even when calling legitimate merchants or issuers, it is not wise to give card numbers over cordless phones because these are exposed to eavesdropping. Cordless phone eavesdroppers may use overheard information to trick a card holder into a scam.