... of audio encoding, including the A-LAW PCM, ADPCM, GSM, G.729 and so on, volume can be adjusted by the ... models: GX02U-1T 1-channel telephone voice box GX02U-1RT 1-channel telephone voice box(recording, response and message) GX02U-2T 2 ... -channel telephone voice box GX02U-2RT 2-channel telephone voice box(recording, response and message) GX02U-1U1T 2 ...