A method for forming a dynamic verification value. The method includes altering a first data string to form a second data string, and forming a first dynamic verification value using at least a portion of the second data string. The first dynamic verification value is used to authenticate a phone in a first transaction. The second data string is used to form a third data string. A second dynamic verification value is formed using at least a portion of the third data string. The second dynamic verification value is used to authenticate the phone in a second transaction.