What OS are you using? In Windows 7, there's a boost in its Sound settings or use the equalizer of your sound card application. I don't use Magic Jack but another VoIP service
from Onesuite and I do those when I need to boost the sound when using my computer to make a call, but rarely as my Logitech headset
is loud enough.
If everything fails try to replace your handset. Maybe borrow one from a friend or neighbor just to check if its handset, if volume still low then its the Magic Jack device itself.