When you're someplace where you can't take a call on you iPhone, you can silence the ringtone quickly. And, if you want to, you can send the call directly to voicemail.
As you're iPhone rings, press the power button to mute the ringtone. Then, you can send the phone call to voicemail by pressing the power button again.
So remember, when you're in a meeting, in class, or in a movie theater (oops!), you have the "power" to silence your phone.