The U-1496 series modems support five voice digitization schemes. Four schemes use the ADPCM algorithm and one uses the advanced CELP (Code Excited Linear Prediction) algorithm to achieve near-phone-quality voice at 9.6 kbps speech data rate. A summary of these four schemes is listed below:
  Digitization       Speech Compression                    Data Rate
   Scheme                Algorithm
    CELP             Code Excited Linear Prediction         9600 bps
    2-ADPCM          ADPCM, 2 bits/sample                  19200 bps
    3-ADPCM          ADPCM, 3 bits/sample                  28800 bps
    3-ADPCM(NEW)     ADPCM. 3 bits/sample                  30720 bps
    4-ADPCM          ADPCM, 4 bits/sample                  38400 bps
where the sampling rate used is 9600 samples per second.
The ADPCM voice modes are supported on all U-1496 series modems. However, the advanced CELP scheme is supported only on the plus version and the U-1496 LCD model.
Silence detection is implemented to eliminate voice data coding in silence period.
The modem also supports simultaneous DTMF (Dual Tone Multiple Frequency) tone detection that allows a user to use the keypad of a tone-dialing, push-button telephone set to control, instruct, or answer the modem.
 Automatic detection of voice, fax, and data.
 Automatic detection of voice, fax, and data.
 ZyXEL Advanced Voice Capability.
 ZyXEL Advanced Voice Capability.