This has happened to me several times in slightly different cases. Several times I've mistakenly placed calls to the wrong people (due to just messing around with the phone).
In a panic, I pressed the "back" button thinking that it will cancel the call. It didn't. I also tried the "home" button. Also no good.
I had to drag down the notifications bar to see the phone call in progress and finally access the red Hang Up button. That worked. The problem is that is is SLOW. And worse, one time the phone would not respond to the button press until after several tries (maybe the CPU was busy on something).
Anycase, that would appear to be a major problem with all these touchscreen-only smartphones - lack of a physical Hangup button.
Is there any workarounds to this? Perhaps some sequence or combo of the existing physical buttons to forcefully hangup an errant call?