Good News! You can silence ringer during an incoming call, if you are using Windows Mobile Smartphone 6.1, with Kavanna or Ricky ROM on a T-Mobile Dash or HTC S620. Follow the discussion here:
You can silence the ringer when a call comes in OR
You just want to "shut the phone up" without declining the call.
Look in the registry and set as follows:
If you dont see these keys, try creating and setting apprpriate values.
With these settings you will need to double-tap the volume strip when you receive a call, and presto!!!