Abstract: A transaction-enabling instrument, such as a credit card, a debit card, or a check, has encoded thereon in machine-readable form a telephone number of a portable wireless communications device, such as a cell phone or a PDA, of the instrument's owner, who is typically the holder of the instrument, and a transaction-authentication code. When the holder presents the instrument to enable a transaction, a reader reads the phone number and code, and an authentication server causes the phone number to be dialed and the owner to be prompted for the authentication code. If the owner provides the code, the server allows the transaction to proceed. If the owner cannot be reached or does not provide the code, the server denies the transaction.