Most quality issues with VoIP have nothing to do with net speed or bandwidth. This is a misnomer. In fact, MagicJack typically only utilizes 87mb/s upstream/downstream data, though I've read some sites recommend 101mb/s speed for best quality. The real quality issues have to do with latency or packet delays, variations in delays called jitter, and packet loss. VoIP is not very tolerate of any of these issues, as it typically utilizes a small buffer.
VoIP generally works best with wired broadband connections such as cable or fibre. Using wireless mobile connection for VoIP can be iffy as mobile data tends to exhibit higher latency and jitter rates overall, which may be attributed to additional system delays.
VoIP also works best when given priority over the net connection. This may be why services such as the Ooma and Vonage perform better quality wise, as they essentially supply their own router/adapters which I believe have Quality of Service(QoS) set to prioritize VoIP. You can do something similar with the MagicJack Plus connected to a router that supports QoS settings.
Though many criticize the consistency in voice quality of devices like the MagicJack or the free Google Voice chat, the fundamental issues are more likely the quality of net connection itself. Something the end user rarely has control over. Bottom line, if you have a poor quality connection, than no VoIP device is going to work.
You really need to assess your net connection quality at each location you're using VoIP. You can find an online VoIP test from sites such as
myspeed.visualware.com. Requires Java installed and enabled in your browser.
