TeamSpeak is freeware for non-commercial users. Commercial users need to buy a license.
Commercial use is defined as companies and their employees installing TeamSpeak.
If the server is installed on a company owned server: you need to buy a license.
If the server is installed by an employee of a company on a computer of a company: you need to buy a license.
If you are asking money to make use of your server: you need to buy a license.
If TeamSpeak is used to as an assistence to your company's processes (be it in any way): you need to buy a license.
For information about commercial use of the TeamSpeak server, pricing information and ordering: mail.