1. A system comprising: one or more computers; and a computer-readable medium coupled to the one or more computers having instructions stored thereon which, when executed by the one or more computers, cause the one or more computers to perform operations comprising: training a probabilistic model using data that reflects whether, given past combinations of (i) state data that references a state of a mobile device and (ii) sensor data that is sensed by one or more sensors of the mobile device, a user of the mobile device did or did not actually initiate a voice input; obtaining, as input data, particular state data that references a particular state of a mobile device, and particular sensor data that is sensed by one or more sensors of the mobile device; inferring, based on applying the probabilistic model to the input data, that the user of the mobile device is likely to initiate voice input; and invoking voice input functionality of the mobile device in response to inferring that the user is likely to initiate voice input.