21. A speaker verification system comprising: at least one computer readable storage medium storing a voice print obtained from a user's utterance of at least one enrollment utterance; a receiver to receive a voice signal of the speaker uttering at least one challenge utterance, wherein the at least one challenge utterance includes at least one word that was not in the at least one enrollment utterance; and at least one controller, coupled to the memory and the receiver, configured to determine whether the speaker is the user based, at least in part, on voice signal and the voice print.
21. A speaker verification system comprising: at least one computer readable storage medium storing a voice print obtained from a user's utterance of at least one enrollment utterance; a receiver to receive a voice signal of the speaker uttering at least one challenge utterance, wherein the at least one challenge utterance includes at least one word that was not in the at least one enrollment utterance; and at least one controller, coupled to the memory and the receiver, configured to determine whether the speaker is the user based, at least in part, on voice signal and the voice print. 24. The speaker verification system of claim 21 , wherein the receiver receives the voice signal of the speaker uttering at least one challenge utterance selected from a challenge vocabulary comprising a plurality of challenge words, wherein the challenge vocabulary includes words having substantial phonetic overlap with the at least one enrollment word used in the at least one enrollment utterance.