15. A computer-readable storage device storing software comprising instructions executable by one or more computers which, upon such execution, cause the one or more computers to perform operations comprising: receiving audio data encoding an utterance, and data specifying a time when the utterance was spoken; determining, for each of multiple communications that were initiated by a user of a mobile device, a time when the communication was initiated or received; determining, for each of the multiple communications, a similarity score based on a similarity between the time when the communication was initiated or received, and the time when the utterance was spoken; determining, for each of multiple contacts associated with the user, a probability associated with the contact based at least on the similarity score for the communications that were initiated or received; weighting a contact disambiguation grammar according to the probabilities; and processing the audio data using the contact disambiguation grammar to select a particular contact.
20. The device of claim 15 , wherein the operations comprise: receiving data specifying whether the mobile device was docked or holstered when the utterance was spoken; determining, for each of the multiple communications that were initiated by the user of the mobile device, whether the mobile device was docked or holstered when the communication was initiated or received; and wherein the respective similarity score is determined further based on a similarity between whether the mobile device was docked or holstered when the communications were initiated or received, and whether the mobile device was docked or holstered when the utterance was spoken.