The aim of the study was to describe the telephone nurses' experiences of encountering callers with mental illnesses. Telenursing services are solely staffed with telenurses, who with the support of a decision support system (CDSS) independently triage callers based on the severity of the main symptoms presented by the care seeker. The system focuses on somatic symptoms, while information regarding mental health and mental illnesses is limited. Information about telenurses' experiences of encountering care seekers with mental illnesses is scarce, despite the increase in mental illnesses in the population. The study used a descriptive design with a qualitative approach. Twenty telenurses were interviewed, and the data were then analysed using inductive qualitative content analysis. The results are elaborated in the following three categories: (i) Experiences of encountering care seekers with mental illnesses; (ii) Experiences of facing difficulties and challenges; and (iii) Experiences of facing dissatisfaction and threats. Encountering care seekers with mental illnesses is metaphorically addressed as 'when all other doors are closed'. Encountering care seekers with mental illnesses was perceived as time-consuming and did not adequately correspond to the resources given by the service. Even though telenurses strive to achieve agreement, there is a collision between human needs and organizational structures. The study pinpoints the lack of resources for and education about mental illnesses and the limitations of the decision system, which needs to be updated in order to provide all care seekers care on equal terms.