I have lost a number of friends to suicide, and as a result I spent a significant amount of time thinking about what could have been done to have helped them before it was too late. In nearly all cases, it was pretty well thought out and not a spur of the moment thing. In some cases, they even took steps to prevent people from discovering their plans. So anecdotally, a suicide hotline would not have helped at all.
Based on some searching though, it seems like there is data showing that it helps on the whole. I guess for people who are having a spur of the moment thought, it might be potentially helpful. However I also found some people saying that seeing the prolific messages about calling the hotline when searching for information about things really pushed them away and somewhat backfired.
I guess ultimately this is a very complex issue with no one size fits all solution. If I ever get to a point where I no longer have to work, this is a cause I would love to work on for the people that tend to think it through more and less spontaneous.