Regulatory agencies seek to balance markets by protecting consumer rights while fostering service provider’s innovation. The Theory of the Marketing Firm (TMF) is a well-grounded theoretical framework for guiding regulatory actions once it emphasises the reciprocal relationship between firms and consumers. This chapter delves into how Brazil’s National Regulatory Agency (Anatel) relied on TMF to map and evaluate consumer relationship practices (CRPs) in telecommunications services. Their research involved a survey identifying the leading Brazilian telecom operators’ main CRPs. It also counted with a mystery shopper approach, where consumers assessed these CRPs based on functional criteria relevant to responsive regulation: safeguarding consumer rights by enhancing access to contracted benefits, minimising punishing events, and broadening consumers’ choices without unduly restricting service providers. Anatel’s case study demonstrates that the functional approach TMF offers is a valuable guide for regulatory agencies in developing evidence-based and theoretically consistent assessments and interventions.