It might be that your headset does not support that function. I have several bluetooth headsets ... some work, others do not. I am currently using an older headset (Motorola H700) with my Verizon Imagio and it works perfectly with Voice Command. In addition, I also use a BlueAnt Z9i and that works as well.
Additionally Microsoft Voice Command do not work on my device after i push defined on my phone key that initiate ms voice command application i only hear error beep and noise in main speaker.
Can you explain that quote above ... I am not quite sure what you are asking. Are you referencing defining a "button" on your device to activate Voice Command? If so, did you assign a button and are having trouble with it working? If so, try a different button to see if it will make a difference.
After you assign a button to activate Voice Command, clicking on that button will activate VC and you will hear a beep and also see a microphone icon appear at the top of the screen. When you do see that, say this to test it: "What can I say". Voice Command should come back with a list of topics.
Jack Cook
http://www.experiencemobility.net http://www.mobilitysite.com