The following is an important write up by Digium. The information is spot on.

Networks are capable of using a protocol named Quality of Service (QoS) to prioritize important traffic types. With an unmanaged switch all network traffic is sent as it is received. No differentiation or fragmentation is assigned to types of traffic or traffic order. This means that as one person is sending a large video file over your network or server, then they will be using up a large portion of the available bandwidth for that application. In turn, it could starve out other applications, such as voice. If someone later picks up the phone to make a call, this call (your voice traffic) will also be transmitted on the same network pipe as the video file.

How to Prioritize Voice Traffic

This is when QoS comes in and helps. In an unmanaged network, the video traffic and voice traffic will interfere with each other, competing for bandwidth. This is why voice calls will sometimes sound great, and why at other times they sometimes sound horrible. When voice quality fluctuates like this, installing QoS-capable switches and routers are the most important first steps to fixing the problem.

With managed switches, your VoIP platform can tell the network that it is trying to transmit highly important voice data, which will be tagged with special QoS markings. If the network supports it, the voice traffic will then take priority over the data traffic. Sure, this may slow the data transfer down, but at least you won’t miss out on any part of your voice call – it will sound crystal clear and you will still be able to transfer all of your data.

Does QoS work over the Internet?

Unfortunately, due to the nature of the Internet, there is no way to guarantee quality of service on an Internet connection. This is simply because the QoS information (or tag) may be stripped off the network traffic (packet) before it even hits the Internet switches and routers.

Tagging Voice Packets

Your VoIP phone system may be able to tag your voice packets with QoS information, and calls inside your company network will sound fine. If you make a call through a SIP provider, or to another site over the Internet, then there is a risk that the voice quality will suffer. There are methods for increasing quality control over the Internet.

Your networking equipment can affect voice quality.

The kind of networking equipment you are using is critical to having a good quality phone conversation. Companies with fewer employees will have an easier job of choosing networking equipment and can shop based on price rather than robust processing and management capabilities because they have fewer networking requirements. There are several reasons that explain this. In a smaller company, you typically have fewer users and less simultaneous voice, video and data traffic. For example, if your company is a small retail store with a few phones, and has a couple of Point of Sale (POS) terminals with no desktop computers, then you won’t be transmitting a significant amount of data. Most data networks of any class are equipped with enough bandwith and resources to handle simultaneous voice and data traffic.