If the quality of the audio is not good and/or appears to cut in and out, the cause of the problem is most likely related to a poor, unstable or slow internet connection.
Have you attempted to communicate (2ways) with this voip line using software such as Express Talk? If you experience audio cut-outs, delays, and/or poor audio quality when you do then the problem is not so much in the software, but rather the connection you are using to access the server providing the VoIP line.
If this is the case and upgrading to a better quality internet connection (eg at least T1 / 1.5 mbps). If this is not a viable option, you can go into IVMs settings for a line and select the preferred quality/bandwidth settings (i.e choose the option that uses lower bandwidth but has lower quality, this way although you won't obtain crystal clear voice that you would on a land-line, you will still be able to understand what is being said without the cutting in and out or sudden drastic changes in audio quality.