The device and method for scrambling/descrambling voice/data in a mobile communication system involves vocoding or bypassing a signal received from a radio channel interval, scrambling the vocoded or bypassed signal using a random number, sending the scrambled signal to a cable channel interval, descrambles the scrambled signal received from the cable channel interval, vocoding or bypassing the descrambled signal, and sending the vocoded or bypassed signal to the radio channel interval, thereby dramatically security of the data in the cable channel interval between the system vocoders and preventing overhearing of the voice.