Let's not forget about the civic duty to call 911 if you suspect that your friend or your neighbor is suicidal. Emergency medicine has experts that can help much more than National Suicide Prevention Lifeline.
This is the problem with how we deal with suicide. People seem to think its their "duty" to take the right to end your life away from you. Police and emergency services will go as far as tie you up and lock you in a padded cell to take this right away.
What we need to is make it clear that suicide is your right. Treatment should be completely voluntary at all times. I would wager that the possibility of being restrained for days or weeks prevents many people from seeking help that could actually have made a difference.
No, it isn't your civic duty to take the right to suicide away from someone. It's your duty as a friend and a human being to be a non-judgmental ear and shoulder. But in the end you have to respect their rights as an autonomous being.
That might be true for some people but not everyone. Do you take into account situation where someone can get treatment and change ones mind? Since you, a non-medical professional, cannot make that judgment it's better to use as a rule to always call.
Emergency visits are not about locking someone away but about figuring out a best treatment. Whether a person will follow the treatment or not that's a different story but at least someone was there for them, to listen, to help, and with means and experience to do it.