What? How was it more ambiguous? There's literally no reason to believe that this was something imagined. And even it if was, it would make it even WORSE sense at that point the idea of excessive plot twists just makes the episode a complete joke.

More like irresponsible, it is absolutely insane in every way to allow cell phones at all in the room. Like if you know that a call could brain fry someone, why the fuck would there not be protocol to lock all belongings somewhere outside of the room?

I agree 100%, the ending actually ruins everything that came before it. It removes any sort of meaningful take away or ambiguity that we could have had by throwing in this extra SIKE for the sake of it. Like, we get it, you're Black Mirror, you have twists. But by adding that one final twist, the episode is ruined.