Background <p>Understanding the factors that shape implementation processes is fundamental to improving outcomes and applicability of interventions. This study reports on part of a process evaluation nested within a cluster randomised controlled trial. It aimed to explore professional perspectives on the challenges and lessons learnt during implementation of a multilevel (patient, practitioner, and service-level) quality improvement intervention developed to enhance telephone-delivered psychological interventions in NHS Talking Therapies services for anxiety and depression (NHS TTad).</p> Methods <p>Qualitative semi-structured interviews with twenty-eight professionals recruited from nine NHS TTad services were conducted. The Consolidated Framework for Implementation Research (CFIR) and Normalisation Process Theory (NPT) were used to inform data collection and analysis. Template analysis was used to exploit the structured approaches of CFIR and NPT whilst retaining the flexibility of thematic analysis to examine the factors perceived to affect implementation.</p> Results <p>Four themes were interpreted from the data: (1) Perceived value of the intervention and its distinctiveness from routine practice; (2) Implementation climate and variation in uptake across services and practitioners’ telephone experience; (3) Need for implementation clarity and leadership support within existing constraints; and (4) Planning for monitoring and continuous improvement throughout implementation. Supportive leadership, planning and execution were important factors, supporting implementation into routine practice. Team members valued the protected time they had whilst in receipt of the intervention, reporting they could reflect on current practices, strengthen relationships and engage in collaborative feedback which supported co-creation of quality improvement plans to enhance telephone-delivered low-intensity psychological interventions. Analysis also revealed limited changes were implemented and sustained over time.</p> Conclusions <p>Successful implementation and sustainability of the intervention across study sites was dependent on active leadership engagement, sufficient time and resources to build a shared understanding of work processes. Collective recognition of the value of the intervention by team members and clear understanding of what changes are needed, who is responsible for them, and the timeline for implementation was considered vital and may have implications for successful implementation nationally in Talking Therapy services. The COVID-19 pandemic significantly influenced the context of implementation, accelerating the adoption of telephone services but also reshaped service priorities and challenges during the study period.</p> Trial registration <p>ISRCTN22583714, Registration date: 01/09/2021</p>