Background
Evidence shows psychological interventions delivered by telephone can be as effective as those delivered face-to-face [The COVID-19 pandemic forced a sudden shift towards remote therapy delivery, with no preparation time in which to train practitioners in telephone-specific skills [Before the implementation of any new and complex health care intervention, the Medical Research Council (MRC) framework [The Theory of Change (ToC) approach is a pragmatic framework, which describes how and why an initiative/intervention works and can be useful in both intervention planning and in empirical testing [The aim of this article is to explore the use of a Theory of Change (ToC) and an implementation framework of analysis to increase the likelihood of complex interventions being effective, sustainable, and scalable. A ToC was developed to represent how changes of a complex intervention, aimed at enhancing the quality of telephone-delivered psychological therapies, were expected to occur to trigger the intended outcomes. We provide a case example of how the development of a ToC has been informed and amended following evidence from the feasibility study reported here. We also describe potential solutions to overcome implementation challenges and improve likelihood of future implementation success.