If armed people are entering 7-11, so I need to reconsider my preferences for meal. Reaching the phone is not a good idea. this may cause them to react if they notice. If they came to kill someone and it wasn't me, then why would I give them the opportunity to kill myself. in such a case, it would make more sense to move away from there without taking attention and then call. if you are far enough away, you can disturb them by sounding the horn and attract the attention of others. Remember, priority is your life, then others. if you die you can't call and warn anyone. If you survive, you can be sure that at least someone calls the police and the ambulance.
There is just a bit difference being stupid or being hero. And those two concepts are generally being mixed. You're not a cop. You are not authorized to intervene in social events. You will not be able to intervene the bad events there But when you call the ambulance, you can become one his hidden hero by preventing one of the wounded from dying.