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Most applications currently do not use encryption, so be aware that it is potentially easy for someone to eavesdrop on your conversations. If security is a concern for you, read the documentation that comes with your VoIP client. Additionally, if you are using a firewall, it should be configured to allow incoming connections from anywhere, so using VoIP also includes taking risks on the level of site security.
11.4.2. What do you need?
11.4.2.1. Server Side
First of all, you need a provider offering the service. This service might integrate traditional telephony and it might or might not be free. Among others are SIPphone, Vonage, Lingo, AOL TotalTalk and many locally accessible providers offering the so-called "full phone service". Internet phone service only is offered by Skype, SIP Broker, Google and many others.
If you want to set up a server of your own, you might want to look into Asterisk.
11.4.2.2. Client Side
On the client side, the applications that you can use depend on your network configuration. If you have a direct Internet connection, there won't be any problems, provided that you know on what server you can connect, and usually that you also have a username and password to authenticate to the service.
If you are behind a firewall that does Network Address Translation (NAT), however, some services might not work, as they will only see the IP address of the firewall and not the address of your computer, which might well be unroutable over the Internet, for instance when you are in a company network and your IP address starts with 10., 192.168. or another non-routable subnet prefix. This depends on the protocol that is used by the application.
Also, available bandwidth might be a blocking factor: some applications are optimized for low bandwidth consumption, while others might require high bandwidth connections. This depends on the codec that is used by the application.
Among the most common applications are the Skype client, which has an interface that reminds of instant messaging, and X-Lite, the free version of the XTen softphone, which looks like a mobile telephone.
However, while these programs are available for free download and very popular, they are not free as in free speech: they use proprietary protocols and/or are only available in binary packages, not in source format.
Free and open VoIP clients are for instance Gizmo, Linphone, GnomeMeeting and KPhone.