A method, medium, and system for masking voice information of a communication device. The method of masking a user's voice through an output of a masking signal similar to a formant of voice data may include dividing the voice data received into frames of a predetermined size, transforming the frames on a frequency axis thereof, regarded as a domain, obtaining formant information of intensive signal regions in the transformed frames, generating a sound signal disturbing the formant information with reference to the formant information, and outputting the sound signal in accordance with a time point when the voice signal is output.