This End Call caused by unable to call or unable to hang up is a general situation to the iPhone 4 or 4s who just being jailbroken running on iOS 5.0.1
Once you make a call, the one you call can receive your call, however if he/she answers, then both of you are unable to hear each other voices.
And then End the call, your screen will display End Call click the Home button but the situation is the unchanged, so you are forced to quit by double-clicking the home button. If you think rebooting your iPhone will resolve the problem, then you are wrong, it still exist.
So this is what you need to do to fix End Call, can not call or can not hang up:
Navigate to Settings >>> General >>> Restore >>> Restore all the settings.
And see you again in another posts.