I just tried searching Google Scholar, and it turned up studies showing effectiveness:
http://onlinelibrary.wiley.com/doi/10.1521/suli.33.4.400.252... - "Independent raters measured callers' suicidality and mental state at the beginning and end of 100 taped counselling sessions. Changes in suicidality and mental state were measured using a reliable rating scale developed for the study. Significant decreases in suicidality and significant improvement in mental state were found to occur during the course of counselling sessions, suggesting positive immediate impact."
http://link.springer.com/article/10.1023%2FA%3A1022269314076... - "Outcome measures showed observer evaluations of decreased depressive mood from the beginning to the end in 14% of calls, decreased suicidal urgency ratings from the beginning to the end in 27% of calls, and reaching a contract in 68% of calls, of which 54% of contracts were upheld according to follow-up data."
In case people don't know this: these hotlines also help people who are concerned about friends and family members, and there are email-based options like http://www.samaritans.org/.
I volunteer as a crisis and suicide counselor for Didi Hirsch (a national suicide line branch) in Los Angeles. Thanks for posting these studies and validating the important work.
http://onlinelibrary.wiley.com/doi/10.1521/suli.33.4.400.252... - "Independent raters measured callers' suicidality and mental state at the beginning and end of 100 taped counselling sessions. Changes in suicidality and mental state were measured using a reliable rating scale developed for the study. Significant decreases in suicidality and significant improvement in mental state were found to occur during the course of counselling sessions, suggesting positive immediate impact."
http://onlinelibrary.wiley.com/doi/10.1521/suli.2007.37.3.33... - "Significant decreases in suicidality were found during the course of the telephone session, with continuing decreases in hopelessness and psychological pain in the following weeks."
http://link.springer.com/article/10.1023%2FA%3A1022269314076... - "Outcome measures showed observer evaluations of decreased depressive mood from the beginning to the end in 14% of calls, decreased suicidal urgency ratings from the beginning to the end in 27% of calls, and reaching a contract in 68% of calls, of which 54% of contracts were upheld according to follow-up data."
In case people don't know this: these hotlines also help people who are concerned about friends and family members, and there are email-based options like http://www.samaritans.org/.