1. A communication interface apparatus which communicates with a voice interactive system, comprising: a first process unit configured to receive voice information and face information about a user, that provides a voice instruction to the voice interactive system, and determine whether the received voice information belongs to a registered user by comparing the received voice information with a user model to calculate a first probability, comparing the received face information with the user model to calculate a second probability, combining the first and second probabilities, and comparing the combined first and second probabilities to a first predetermined threshold; a second process unit configured to, in response to determining that the received voice information belongs to the registered user, receive the face information, and determine whether an attention of the user is on the voice interactive system based on the received face information, wherein the attention of the user is determined by extracting, from the received face information, first information of a direction of eyes of the user and second information of a direction of a face of the user, combining the first information and the second information based on a weighted factor, and comparing the combined first and second information with a second predetermined threshold; a third process unit configured to, in response to determining that the attention of the user is on the voice interactive system, receive the voice information, analyze a meaning of the received voice information, and determine a consistency of the received voice information based on the analyzed meaning and a dialog model that represents conversation flow on a basis of a situation, and a transmitter unit configured to transmit the voice information to the voice interactive system to perform a specific task according to the voice information, in response to determining that the received voice information belongs to the registered user, the attention of the user is on the voice interactive system, and the consistency of the received voice information is held.