QoS is a router function to give priority to some internet traffic at the expense of other traffic. For example, giving higher priority to VOIP (voice) traffic so web browsing and file transfers don't interfere. A basic primer on QoS is available.
QoS can dramatically improve voice quality, fixing the oft-reported "choppy voice" problem. However, if not configured properly, it can make things much worse.
There are a few ways to obtain the benefits of QoS. They are described below.
QoS and Tomato
See this How-To: Configuring the free Tomato router firmware QoS for MagicJack.
Some consumer routers provide (often minimal) QoS capabilities. The more detailed how-to articles (above) may help you understand what to look for, or how it should be configured.
QoS with no pain
For those who don't want to get too involved in true QoS, see the following simplified solutions. Be aware that these solutions sacrifice flexibility and efficiency for ease of use. Router firmware provides finer control over prioritization, and may have more advanced capabilities like regulating incoming traffic (which, as described in the Tomato QoS How-To, isn't really QoS but a custom feature of the firmware).
Hawking Broadband Booster
Hawking Broadband Booster is a "QoS-in-a-box" product which automatically configures itself. MagicJack users have reported good results using this.
A software solution is appropriate if you have no other computers sharing your network connection.
Netlimiter commercial software tool.
cFosSpeed commercial software tool.