Even though the iPhone is based on soft keyboards, still you can try these troubleshooting steps that come in handy with another external buttons of your iPhone:

Force quit a program in iPhone.

Press and hold the Home button for six seconds to force-quit a program that seems to be stuck.  

Hard Reset iPhone.

If the entire iPhone locks up—it can happen, press and hold both the Home button and the Sleep/Wake switch for eight seconds. You’ll see the screen go black, and then the Apple logo appears as the iPhone reboots. If you really like your iPhone, please see here for more tips about your iPhone. See the 25 Life saving tips for using iPhone. Notify me of follow-up comments by email. Notify me of new posts by email.

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