This article is from the Credit cards and Consumer Credit FAQ, by adams@spss.com (Steve Adams) with numerous contributions by others.
For Discover, it's (800) 347-2683, a/k/a 800-DISCOVEr.
For American Express, it's (800) 528-4800 for green card, or for
gold card (800) 327-2177.
For Visa and MasterCard, each issuing bank handles service of its
own customers. First check the obvious: the number may be printed
somewhere in your bill, or on a page in the packet of stuff the card
company sent you when you enrolled. Or, if it's a local bank, check
the white pages of your phone book. If the bank is not local, try
(800) 555-1212 (it's free) to find if the bank maintains an 800
number; many do. Alternatively, larger public libraries have
banking directories and can probably give you the information at the
reference desk or by phone.
 
Continue to: