Background: People with palliative care needs and their carers often rely on out-of-hours services to remain at home. Policymakers have recommended implementing telephone advice lines to ensure 24/7 access to support. However, the impact of these services on patient and carer outcomes, as well as the health care system, remains poorly understood. Aim: To evaluate the clinical- and cost-effectiveness of out-of-hours palliative care telephone advice lines, and to identify service characteristics associated with effectiveness. Design: Rapid systematic review (PROSPERO ID: CRD42023400370) with narrative synthesis. Data sources: Three databases (Medline, EMBASE and CINAHL) were searched in February 2023 for studies of any design reporting on telephone advice lines with at least partial out-of-hours availability. Study quality was assessed using the Mixed Methods Appraisal Tool, and quantitative and qualitative data were synthesised narratively. Results: Twenty-one studies, published 2000–2022, were included. Most studies were observational, none were experimental. While some evidence suggested that telephone advice lines offer guidance and reassurance, supporting care at home and potentially reducing avoidable emergency care use in the last months of life, variability in reporting and poor methodological quality across studies limit our understanding of patient/carer and health care system outcomes. Conclusion: Despite their increasing use, evidence for the clinical- and cost-effectiveness of palliative care telephone advice lines remains limited, primarily due to the lack of robust comparative studies. There is a need for more rigorous evaluations incorporating experimental or quasi-experimental methods and longer follow-up, and standardised reporting of telephone advice line models and outcomes, to guide policy and practice.