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: establishing, as input data, state data that references a state of a mobile device and sensor data that is sensed by one or more sensors of the mobile device; applying a rule or a probabilistic model to the input data; inferring, based on applying the rule or the probabilistic model to the input data, that a user of the mobile device is likely to initiate voice input; and invoking one or more functionalities of the mobile device in response to inferring that the user is likely to initiate voice input.