iPhone Basics: Silence Your Ringer & Send A Call To Voicemail
If a call comes in and you want to mute your ringer, pressing the Sleep/Wake button or one of the volume buttons will immediately silence your phone. If you’d like to send a call directly to voicemail, you can press the Sleep/Wake button twice to decline it. You can also do this if you’re wearing an iPhone headset by quickly pressing the microphone button twice.
Now you can selectively ignore people you don’t want to talk to. Handy!