You need to check following points:

  1. Signal quality – The quality of the signal must be in the range -65dBm and -85dBm in the module without call activity.
    You can check the quality of the signal in the configuration of the 2N® VoiceBlue Next (Gateway control –> Module control) or by using the command at&qall from the hyperterminal or putty.
  2. Switch setting – Check if the port is in the mode of FULL DUPLEX. Half duplex does not make sense for UDP RTP packet. In case there is just one call via this port, the quality can be satisfying, but with more call traffic the issue with voice quality appears.

The best tool how to check if it is caused by defective 2N® VoiceBlue Next or another things is to connect the 2N® VoiceBlue Next to the PC and make a direct SIP call from software phone as SJ phone. Use the called number in this format: sip:phonenumber@ipaddressofVoiceBlueNext.