Have you tried port forwarding? Go into your router administration settings and apply these forwards to the computer's IP address you use MagicJack on.
UDP Traffic
Port 5070 for Registration(This is only used for First Time Setup)
Port 5060 and 5061 for SIP Messaging
If you don't know how to do that: (Windows Solution)
Go to "Start"
Type "cmd" into the search box and press enter(For Windows XP go to "Run")
Type "ipconfig" (You may need "ipconfig/all")
Find the connection device(i.e. Ethernet adapter Local Area Connection 1)
Find "IPv4"
Use that IP address.
Hopefully this helps!
-UrFriendlyVirus