Objectives To examine the skills and expertise required and used by non-clinical call-handlers doing telephone triage and assessment, supported by a computer decision support system (CDSS) in urgent and emergency care services. Methods Comparative case study of three different English emergency and urgent care services. Data consisted of nearly 500 hours of non-participant observation, 61 semi-structured interviews with health service staff, documentary analysis, and a survey of 106 call-handlers. Results Communication skills and ‘allowing the CDSS to drive the assessment’ are viewed by the CDSS developers and staff as key competencies for call-handling. Call-handlers demonstrated high levels of experience, skills and expertise in using the CDSS. These workers are often portrayed simply as ‘trained users’ of technology, but they used a broader set of skills including team work, flexibility and ‘translation’. Call-handlers develop a ‘pseudo-clinical’ expertise and draw upon their experiential knowledge to bring the CDSS into everyday use. Conclusions Clinical assessment and triage by non-clinical staff supported by a CDSS represents a major change in urgent and emergency care delivery, warranting a detailed examination of call-handlers’ skills and expertise. We found that this work appears to have more in common with clinical work and expertise than with other call-centre work that it superficially resembles. Recognizing the range of skills call-handlers demonstrate and developing a better understanding of this should be incorporated into the training for, and management of, emergency and urgent care call-handling.