This can be caused by a variety of problems. The most common is poor Internet connection quality or lack of bandwidth.

Very often what is sold as a "High Speed" or "Broadband" connection is not up to the task of transmitting voice packets in a timely or reliable manner. Problems such as high latentcy (slow delivery of packets) or packet loss (actual dropping of voice data) can be seen from time to time on some provider's networks. These problems will be heard by you on your IP phone call long before they are noticed for other data activities like checking email or web browsing.

To track down where the problem is coming from, you can use packet tracing software to reveal network bottlenecks and get them reported to your ISP. We recommend a product called "Ping Plotter". You can download a free trial version at http://www.pingplotter.com