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It sounds like that End call button just does not want to play nice. Here is what I recommend to help troubleshoot the issue.
Steps 1, 3, and 5 of the iPhone Troubleshooting Assistant found here http://www.apple.com/support/iphone/assistant/phone/. If you get to step 3, close all the apps that are running. Here is step 1 to get you started:
To restart iPhone, first turn iPhone off by pressing and holding the Sleep/Wake button until a red slider appears. Slide your finger across the slider and iPhone will turn off after a few moments.
Next, turn iPhone on by pressing and holding the Sleep/Wake button until the Apple logo appears.
Is iPhone not responding? To reset iPhone, press and hold the Sleep/Wake button and the Home button at the same time for at least 10 seconds, until the Apple logo appears.
If your device does not turn on or displays a red battery icon, try recharging next.
All the very best,