Just to add to Greg's good info;
About Choosing Headsets and Handsets for Cisco IP Communicator
Even though internal testing of third-party headsets and handsets for IP Communicator has been performed, Cisco does not certify or support products from headset and handset vendors. Because of the inherent environmental and hardware inconsistencies in the locations in which IP Communicator is deployed, there is no one best solution that is optimal for all environments. Cisco recommends that customers test the headsets and handsets that work best in their environment before deploying a large number of units in their network.
The primary reason that support of a headset or handset would be inappropriate for an installation is the potential for an audible hum, either a hum heard by the remote party, or by both the remote party and the IP Communicator user. Causes for the hum range from electrical lights near the PC to the PC power source itself. In some cases, a hum experienced by a user using an audio device that is plugged directly into the PC USB port may be reduced or eliminated by using a powered USB hub.
In some instances, the mechanics or electronics of various headsets can cause remote parties to hear their own voices echo back when they speak to IP Communicator users. The IP Communicator user will not be aware of the echo.
Finally, some analog headsets do not match the electrical characteristics for which some soundcards are designed. The microphones on such headsets are frequently too sensitive, even when the input levels in IP Communicator are reduced to their lowest values; the voices of users on such headsets will sound distorted to remote parties.
It is important to ask IP Communicator users whether a particular headset sounds good to them. In addition, remote parties should be queried as to the reception from IP Communicator when using a particular headset.
Testing performed in Cisco labs has shown that the Plantronics DSP 300 and Plantronics DSP 400 have performed well with IP Communicator, but it remains the customer's responsibility to test the headsets in their environment.
From this 1.1(5) doc;
Cisco IP Communicator Guidance for Headsets and Handsets
Phoenix Audio Technologies: http://www.phnxaudio.com