Please go to Settings->Accessibility-> "Start button stops the call" check box. Check that if it's unchecked or uncheck if it's checked. This will reset the layout of the call screen (at least it did in my case) and the buttons will be back - you miss all the buttons not only the end call button - exactly as I did.
Note:"Start button stops the call" - I do not know what exact text you have there, as I have the UI in Romanian and I have just translated the Romanian text to English, anyway, at least in my case it is the only checkbox in that settings screen, so you cannot possible miss it.
Hope it helps.