1. A method comprising: receiving unconstrained input speech from a user; converting, via a processor, only the unconstrained input speech corresponding to single digits into a string of words, wherein each word in the string of words is modeled using a three segment structure comprising a plurality of heads and a plurality of tails; converting the string of words into a sequence of digits using classes of rules and according to an acoustic model database in which Markov models characterize acoustic features of numeric words; comparing the sequence of digits to a plurality of valid sequences of digits, to yield validity information; and providing the validity information to a device associated with the user.