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Regardless of what you’re doing on your iPhone, when an incoming call is received, everything else is put on hold and the Phone app launches, unless the iPhone is turned off or in Airplane mode, in which case calls automatically go to voicemail.
To control the volume of the ringer, press the Volume Up or Volume Down buttons on the side of your iPhone; or to turn off the ringer (which causes the phone to vibrate when an incoming call is received), turn on the Mute button on the side of the iPhone (assuming that you have this featured turned on from within Settings).
Tip
While your iPhone is still ringing, to send an incoming call to voicemail (without answering it), press the Power button on the top of the phone once, or tap the Decline icon displayed on the screen.