UC500.com

It looks basically like the 7921G with the following enhancements:

• Support for Bluetooth v2.0 headset profiles
• Hermetically sealed phone/display that is IP54 rated
• Ruggedized industrial design is compliant with military 810F standard with a rubber casing to shield the phone from damage
• A more compact form factor gives you an IP phone that is easier to hold .

There is more info available here. . .
http://cisco.com/en/US/products/ps9900/index.html