TeamSpeak sofware consists of both client and server software: all you need to get Voice over IP working in your business environment. The software is robust enough to handle thousands of connections at the same time.
TeamSpeak is free for non-commercial use and can be administrated by an easy to use web interface. If you want to use TeamSpeak software in a commercial way you must purchase licenses. You can either 'rent' teamspeak servers to resell slots/connections to your own customers or you can buy a license for a whole year and then you can put the software on your own servers. This service is aimed at businesses wanting to reduce costs on teleconferencing or long distance telephone charges.
TeamSpeak 2 can be run on Windows and Linux computers. A processor with at least 166MHz is needed and 64MB RAM. And ofcourse you will need a fast internet connection (aDSL or sDSL).