The voice quality of a VoIP phone call should be at least as good as what the PSTN can provide. This is true as long as enough bandwidth is available for the call. If bandwidth is somehow constrained, quality of service can be affected. In a condition where adequate bandwidth is not available, the call can begin to degrade in quality not unlike a bad cell phone connection.
Quality of service is controlled in several ways. Calls within your LAN environment should not get degraded if the network is well designed.
If you extend your VoIP calls outside of your LAN over a WAN or out on the Internet, then quality of service does become an issue. Routers and switches are available that can prioritize voice packets to maintain good quality of service. On the Internet, there is really very little that you can control when it comes to bandwidth. In real world experiences, the Internet does seem to be able to provide a good option for VoIP calls with surprisingly good quality when using broadband connections.