Background: Telephone-administered cognitive-behavioral psychotherapy (T-CBT) can effectively treat patients with depressive symptoms. Introduction: We investigated whether adding letters (via postal mail) to T-CBT reduces depressive symptoms and increases response and remission. Additionally, we assessed whether T-CBT reduced all patients' symptoms in the first depression-specific T-CBT sample in German healthcare. Materials and Methods: Primary care patients were randomized to T-CBT with versus without letters. All received 1 face-to-face and 8-12 telephone-administered sessions. An intention-to-treat sample was analyzed. Between-groups differences in symptom change and the total sample's symptom change were computed using linear mixed models with group as fixed effect, referring general practice as random effect and several covariates. Differences in response and remission were assessed using logistic regressions. Results: Fifty-nine patients were referred to T-CBT and randomized. Twenty-six patients actually participated in T-CBT with letters and 21 without letters. The groups did not differ significantly regarding symptom change (Patient Health Questionnaire [PHQ-9]) from baseline to end: T-CBT without letters showed 1.05 points greater reduction (95% confidence interval: -4.72 to 2.62; p = 0.56; Cohen's d = -0.12) (adjusted mean change). The groups did not differ significantly regarding symptom change from baseline to 6-month follow-up nor odds of response or remission. The total sample's PHQ-9 showed significant adjusted mean reduction from baseline to end of T-CBT and to 6-month follow-up. Discussion: Additional letters did not lead to greater symptom reduction. Overall results for the first German T-CBT intervention for depression appear promising but require further assessment using a control condition. Conclusions: Additional letters do not appear to enhance the effectiveness of T-CBT.