Embodiments of the invention provide for speaker verification on a communication device without requiring a user to go through a formal registration process with the issuer or network. Certain embodiments allow the use of a captured voice sample attempting to reproduce a word string having a random element to authenticate the user. Authentication of the user is based on both a match score indicating how closely the captured voice samples match to previously stored voice samples of the user and a pass or fail response indicating whether the voice sample is an accurate reproduction of the word string. The processing network maintains a history of the authenticated transactions and voice samples.