Introduction <p>COVID-19 pandemic restrictions accelerated the use of mHealth interventions to maintain continuity of care for youth living with HIV (YLH). Evaluation of mHealth implementation during the pandemic can inform optimization of use among healthcare workers (HCWs) beyond emergencies. This study identified key influences on perceived acceptability, feasibility, and reach of phone delivery of an HIV care transition intervention, the Adolescent Transition Package (ATP), a booklet detailing themes on HIV disclosure and transition readiness, and relevant assessment tools, to support care transition for Kenyan YLH.</p> Methods <p>We conducted 50 semi-structured interviews with a purposively selected subset of frontline (HCWs) from intervention sites participating in the Adolescent Transition to Adult Care for Adolescents living with HIV (ATTACH) study evaluating the ATP between June 2020 and January 2021. In this hybrid effectiveness-implementation cluster randomized trial, we nested a mixed methods study. We conducted and recorded interviews, and transcribed verbatim. Applying the Consolidated Framework for Implementation Research (CFIR), we conducted a directed content analysis to evaluate determinants of acceptability, feasibility and reach of ATP phone delivery. We also documented all attempted and completed calls, including call characteristics such as duration, and summarized these data using descriptive statistics.</p> Results <p>Out of 1,444 call attempts, 82% successfully reached YLH being contacted. Providers successfully discussed the ATP during 79% of calls where YLH were reached. Determinants influencing perceived acceptability, feasibility, and reach of phone delivery of the ATP were concentrated in intervention characteristics, inner and outer setting CFIR domains. Overall, HCWs felt phone calls were acceptable and feasible for reaching most youth. However, limited ability to assess client comprehension and inability to share ATP booklet pictures in real-time negatively influenced acceptability. Within the inner setting, few phones at the clinic, missing or inaccurate contact information, the composition and organization of clinic teams, and HCW self-efficacy drove perceptions of feasibility. HCWs’ recognition of limited phone ownership and incomplete disclosure to others in the home environment were recognized outer setting barriers to reach and acceptability. In contrast, the ease of implementation, convenience, and flexibility afforded by calls positively influenced perceived acceptability and feasibility, and reach. Views on implementing calls beyond the pandemic were mixed, HCW noting that long-term access to clinic phones would be essential for sustained feasibility.</p> Conclusion <p>Delivery of the ATP by phone was an acceptable and feasible strategy to reach YLH during COVID-19. Phone call feasibility was limited by low phone ownership but enhanced when there were strong collaborations and communication between clinic staff. Acceptability of the phone call strategy was driven by its ease, efficiency and capacity to reach many YLH but challenged by the inability to assess and ensure client comprehension. Future mHealth initiatives may improve the implementation of calls by addressing phone access, comprehension and privacy-related constraints to maximize benefits.</p>