If you're dealing with someone who's depressed and/or suicidal, the last thing you should say is, "Call me. I'm here for you." As Alida Brandenburg states in her article, "Stop Telling Depressed People to Call You": "The problem with depression is that you feel so utterly alone, so hopeless, that reaching out for help feels not only like an intense effort, but even agonizing or unwelcome. Your depression tricks you into perpetuating your own pain."
The best course of action when engaging with a depressed person is to treat them "like you would with anyone who is sick or in need." You would never tell a loved one dying of cancer, "Give me a call if you need anything!" You would just do it. You would find out what their needs are and deliver on them. "They need you. Don’t wait for them to ask. They can’t. They are so underwater that it’s cruel to place that additional burden on them."
This is what saving a life is all about; you are giving your depressed friend the love and support that they need but feel too ashamed or guilty to ask for. Given if the roles were reversed, they'd do the same for you.