Which Magic Jack are you using ???
Direct connect (Doesn’t need the computer) or, the old one which is plugged into the computer.
Make sure you have the latest drivers installed.
Make sure that the firewall allows the port that MJ uses.
I’ve been using the Magic Jack Plus version for over a year as my home phone provider without any problems.
I also use the free version of avast! so there isn’t any avast! firewall interaction.
The following may be of help to you:
http://answers.microsoft.com/en-us/windows/forum/windows_7-hardware/firewall-problems-with-magic-jack-not-the-router/28d5f3b7-9154-e011-8dfc-68b599b31bf5?msgId=d97dd584-c854-e011-8dfc-68b599b31bf5