Strange question, but what does the hotline actually do? Supposing you call them with serious suicidal ideation, do they dispatch police? Direct you to a shrink? Just talk?
Under the understanding that suicide is an act of spontaneity and not always highly determined, a hotline gives the amount of time necessary for an acute urge to pass. Depending on the source, it also informs people where and how to get help.
Of course, much of the effects relies on the skill of the folk on the other line. But it is always extremely important to have something, anything, interrupt what is often a compulsion that passes.
1-800-273-8255