Purpose – Telephone crisis services are increasingly subject to a requirement to “prove their worth” as a suicide prevention strategy. The purpose of this paper is to: first, provide a detailed overview of the evidence on the impact of telephone crisis services on suicidal users; second, determine the limitations of the outcome measures used in this evidence; and third, suggest directions for future research. Design/methodology/approach – MEDLINE via Pubmed (from 1966), PsycINFO APA (from 1967) and ProQuest Dissertation and Theses (all to 4 June 2015) were searched. Papers were systematically extracted by title then abstract according to predefined inclusion and exclusion criteria. Findings – In total, 18 articles met inclusion criteria representing a range of outcome measures: changes during calls, reutilization of service, compliance with advice, caller satisfaction and counsellor satisfaction. The majority of studies showed beneficial impact on an immediate and intermediate degree of suicidal urgency, depressive mental states as well as positive feedback from users and counsellors. Research limitations/implications – A major limitation pertains to differences in the use of the term “suicidal”. Other limitations include the lack of long-term follow-up and of controlled research designs. Future research should include a focus on long-term follow-up designs, involving strict data protection. Furthermore, more qualitative research is needed in order to capture the essential nature of the intervention. Originality/value – This paper attempts to broaden the study and the concept of “effectiveness” as hitherto used in the literature about telephone crisis services and offers suggestions for future research.