In some (most?) roms, you can turn on and off the noise reduction during a phone call by using the menu button - menu option is called "Noise Reduction On/Off". With the noise reduction "on" the icon is shown, when it is "off" the icon disappears.
The reason I said "some roms" is because I couldn't see that option in the CheckROM RevoHD™ V6.
You can only change the noise reduction during a phone call, not while dialling or ringing. Although you can turn it on/off during the call, it will not remember the state... it will always start "off" (the original Phone.apk will always start "on").
And no, I didn't delete anything. Just changed a 1 ("on") to a 0 ("off"). The smaller apk size is due to the compression of the classes.dex.
I used the hex editor 0xED for Mac OSX.