A voice recognition system has a vehicle unit, which is a communication terminal, and a voice recognition server. The voice recognition server recognizes a voice signal received from the vehicle unit. A sound characteristic of a communication channel for providing communication between the vehicle unit and the server requires to be adjusted so that the server properly recognizes the voice signal. A test pattern signal is used for adjusting the sound characteristic. The vehicle unit adjusts the sound characteristic based on the test pattern signal. Therefore, the server does not require a large database for the sound characteristic adjustment because it is performed by the vehicle unit.