Troubleshooting for Nokia Purity Pro Wireless Stereo Headset by Monster.
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Why are there no power or Active Noise Cancellation (ANC) keys? - Nokia FAQ
Switching the headset or ANC on or off are built in to the mechanics of the headset. Folding the headset open switches it on, and the proximity sensors on the earpieces switch ANC on when you put the headset on.