I received feedback from our readers regarding the article of MagicJack that I wrote last October 10, 2008 (Cost Effective VOIP) through search engine archive. They said that it is working intermittent for them, sometimes ok and most of the time they can't establish a good connection.
If you are not using a firewall, even running a Windows XP or Vista operating system you should be able to make and receive calls. I've tested this type of setup.
Now with the Firewall device on your network, you need to open the following ports for you to make and receive calls successfully. These ports are listed requirements from MagicJack website.
UDP Port 5060 inbound/outbound
UDP Port 5070 inbound/outbound
I open this additional UDP ports (UPD Port 10000-65535 inbound/outbound) to make a consistent calls. So far, all incoming and outgoing calls are working for me. I haven't check the security vulnerabilities of opening these UDP ports. I have Internet cable and using dynamic external IP Address. In my humble opinion, UDP port attack in dynamic IP address is minimal or maybe not possible. I will check and make a follow up if I find something interesting to share.
Cheers for now.