There are several ways to turn the ringer off on an iPhone. You can either set the phone to "vibrate" in the settings menu, turn the volume all the way down or turn on the mute switch. To turn the ringer back on, you need to reverse whichever action you took to turn it off in the first place.
Tap on the "Settings" application that came preinstalled on your iPhone.
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Tap on "Sounds."
Move the "Vibrate" slider from "On" to "Off." Reverse the process to turn the vibrate feature back on.
Move the "Change with Buttons" slider -- located under "Ringer and Alerts" -- from "Off" to "On." This allows you to control the volume of the ringer through the volume buttons on the side of the phone.
Tap on "Ringtone," located under "Vibrate."
Tap on any ring tone to set it as your default.
Press the Home button -- located on the front of your iPhone near the bottom -- to exit settings.
Move the mute switch -- located on the top left side of your iPhone -- towards the screen to take the phone off mute. A box pops up displaying an image of a bell. If there is no line through the bell, it means the ringer is on. If there is a line through the bell, it means the phone is on mute.
Press the top volume button -- located under the mute switch -- several times to increasing the ringer's volume. Press the lower volume button to decrease the sound.