I guess it happened to you at least once - you were just about to end a call, but your friend did it first. Now instead of the red end call button you automatically pressed text and were redirected to the sms app.
There's an easy way to avoid it - just move the text button anywhere else on the screen. To do it, go to themes, click on the current theme, and in edit screen swipe left / right to switch to the screen during a call. Then drag the button wherever you want.