The Cisco Unified IP Phones 7970G/7971G-GE are full-featured telephones that provide voice communication over an Internet Protocol (IP) network. They function much like digital business telephones, allowing you to place and receive phone calls and to access features such as mute, hold, transfer, speed dial, call forward, and more. In addition, because Cisco Unified IP Phones are connected to your data network, they offer enhanced productivity features, including access to network information, XML applications, and customizeable features. The phones also support security features that include file authentication, device authentication, signaling encryption, and media encryption.
This URL should help you:
http://www.cisco.com/en/US/products/hw/phones/ps379/products_administration_guide_chapter09186a008079978a.html
