- It is against the merchant agreements of MC, Visa, and AMEX, for a vendor to require you to provide your phone number, home address, or other personal information for credit card transactions. In fact, some states make it illegal for them to require it. (It’s not illegal to ask, but it is if they refuse to process the transaction without that information)
- Credit card numbers conform to the Luhn algorithm, which is just a simple checksum test on the number. What you do is start from the right and double each second digit (1111 becomes 2121), then add them all together, and you should end with a number evenly divisible by ten. If it doesn’t, it’s not a valid credit card number.
- The first digit of the number is the Major Industry Identifier. 1/2 are for airlines, 3 is for travel/entertainment, 4/5 for banking and financial, 6 for merchandizing and financial, 7 for petroleum, 8 for telecommunications. 0 and 9 are for other assignments but you’ll likely never see them. If you look at an American Express card, you’ll see it starts with a 3, a throwback to their travel/entertainment roots.
One of my own: If your card is stolen and used, don't panic. I've been through it twice and it worked out fine each time. Be patient while they get your account straightened up. Keep an up-to-date list of automatic charges (satellite company, cell phone, etc.) that go to each card to make this process easier.