First you want to check your DSP Statistics and see information on Jitter and dropped packets. This will give you an idea of the network conditions.
This box doesn't seem to give you too much in the way of flexible QoS settings. To try to improve the situation on the transmission side look at changing the Frames Sent Per Packet.
You'll likely want to use G.723.1 compression if you can. The manual doesn't explain how to configure this so you might need to contact technical support. You'll also want to disable voice activity detection (VAD) as this often causes clipping of speech.
This was first published in March 2004