Are you saying there is no end call button because your screen isn't waking up when you pull the phone away, or because your going to your home screen during the call and don't have the phone app open when your call has ended? There is most certainly an end call button during all calls and I don't think you can turn it off, but here are some things to try or consider;
If your phone won't wake when you pull it away from your ear try cleaning your sensors, upper left next to the speaker.
If you are going to your home screen during calls you can pull the notification bar down and at the top of the list it shows you are on a call. To the right of that there is either a red x or red phone icon that will end the call.
Alternately if you go into your phone app, and then go to settings there is an option to end calls with the power button. Now I'm not sure if selecting this leaves everything the same and just allows you to end the call in an additional way, or if it changes the behavior of the phone and takes away the end call button at the bottom of the phone app and replaces that option with the power button end call option. Either way the notification option should not be changed. You may also want to check your settings in the phone app and see if you have it checked already. If it is try unchecking it and see if your end call button comes back. That's if none of the other items mentioned are true. i.e. dirty sensor, still in the phone app.
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