Although increasingly deployed worldwide, human trafficking hotlines are sorely under-researched. Situated within a complex systems framework, we conceptualise such helplines as both a product of and an influence on broader anti-trafficking ecosystems. Taking the UK as a case study, we undertook exploratory analysis of potential ‘modern slavery’ cases (n=3,613) reported to a major independent anti-trafficking helpline. We examined who seeks help, why and what follows. Contrary to stereotypes, relatively few cases involved sexual exploitation. Many case characteristics varied significantly by exploitation type. Reports about car washes and beauty services heavily influenced overall trends, likely reflecting intense public focus on these sites. Most cases involved adults. Although people self-reporting exploitation are the core target audience, only around 1 in 10 cases derived from self-reports (with higher rates for domestic servitude). We show how third-party reporters vary in their proximity to the people about whom they raise concerns – who themselves may or may not self-identify as victims and/or welcome intervention. Findings around onward action both show a whole-systems response to addressing complex needs and raise difficult tensions around risks of police involvement. Our key contributions include showing what can (and cannot) presently be assessed from such helpline data, proposing a future research agenda and providing a tangible illustration of what it means to theorise helplines as a part of a complex system of anti-trafficking activity. We highlight how their central goal of victim support can be enabled and constrained by wider policies, funding decisions and other structures.