I haven't thought hard about this, but my guess is suicide hotline volunteers aren't that stellar and you could counterfactually save 1-5 more lives per day by volunteering and being great at your job. I'm making some low-confidence guesses about the number of calls you would get per day.
If you exclude heavy-tailed bets I think the current cost to save a life is something like $3000 - $6000, so you're saving more lives than if you earn to give and donate to malaria charities.
Is there a reason this isn't recommended more?
This answers my question, and I'm now pretty convinced being a volunteer is not that impactful. Thank you!