I get a lot of spam calls, but want to silence the ringer without tipping the caller off that I did it. If you want to make an incoming call stop ringing without answering or letting the caller know you hung up, then press one of the volume buttons. If you press the menu button, power button, or on-screen "Decline" button, then the caller will immediately be sent to voicemail (or whatever is in its place), and if it happens soon enough after they dialed you, they will know that you intentionally declined their call. If you use one of the sound buttons, then the caller will continue ringing, but your phone will stop making the ringing sound.