I'm thinking of a situation when the phone, for example, is not enough for them. What if the don't leave you alone after that? What if you're a girl and they are going to try to rape you? What I'm thinking of is not about a "fancy" help-me-I'm-being-mugged "duck-face" pose, but actually a face pattern which, simple enough, could offer assistance in a difficult situation. What if they take away the phone and I'm left with no change of calling for an ambulance?

You are right, though, that this requires some "friction" and probably some self control.