When using the vanilla CC module, Zen Cart does not send your credit card number to anyone for validation. It uses built-in logic to evaluate the card's validity. If you google for MOD10 or Luhn algorithm, you'll see the formula/concept used to check a credit card number for validity.
Zen Cart uses this MOD10 approach combined with the list of accepted cards you have configured in Admin->Configuration->Credit Cards. If the number fails validation or is a card-type that is not accepted based on your settings, it will be rejected with the error message you mentioned.
So, testing with your own personal number is a good approach. Here's an acid test: If you're not comfortable using your own CC number on your own website, why would you expect customers to do that either ? Your fear that it was transmitted to someone else is valid. That's also the reason why the card digits are split up before being emailed, and are not stored in your store in their entirety.
There are other test numbers you can use (after removing dashes), such as:
Visa#: 4111111111111111
Visa#: 4007000000027
MasterCard: 5431-1111-1111-1111
MC#: 5424000000000015
Discover#: 6011000000000012
Amex: 341-1111-1111-1111
Discover: 6011-6011-6011-6611
AMEX#: 370000000000002
Visa#: 4222222222222
You might also note that Zen Cart uses these Credit Card Prefix Patterns (and more) to detect which kind of card your customer has entered.
Visa: 13 or 16 numbers starting with 4
MasterCard: 16 numbers starting with 5
Discover: 16 numbers starting with 6011
AMEX: 15 numbers starting with 34 or 37
There are many other articles about the
anatomy of credit card numbers
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