Device Guides
Apple iPhone 4
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iOS is the operating system software installed on your iPhone. To find out which version you have: Tap Settings, Tap General, Tap About, and view the version number.

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How to
Turning call waiting on my mobile phone on or off
If you want to answer an incoming call during an ongoing call, you need to turn on call waiting. Follow these easy instructions.
Tap Settings.
Tap Phone.
Tap Call Waiting.
Tap the indicator next to Call Waiting.
Depending on the current setting, the function is turned on or off.
Tap the Home key to return to standby mode.