Objective: In this randomized controlled trial, we evaluated the effectiveness of a telephone-delivered intervention based on the Health Action Process Approach (HAPA) after discharge from inpatient rehabilitation to address behavior change, emotional status, and glycemic control in patients with Type 2 diabetes. Design: In a German rehabilitation center, 249 patients with Type 2 diabetes were separated into randomized groups, either a 12-month telephone follow-up support group or the usual care group. The counselor identified personal target areas and intervention modules and developed with the patient an individualized action plan for the telephone support. To enhance motivational processes, they used motivational interviewing techniques. Counselors called patients monthly to support the implementation of the personal plans into the patients’ daily routines and to screen for emotional problems. Assessments measured exercise, diet, medication adherence, psychological variables, body mass index, HbA1c, and cardiovascular risk. Results: Twelve months after inpatient rehabilitation, the telephone group’s rate of physical activity rose by 26% compared with the usual care group’s 10%. Patients in the intervention group exhibited greater improvements in terms of their illness burden, psychological well-being, and depression. HbA1c fell in the telephone group but increased in the usual care group (−0.68% vs. 0.12%). The intervention group’s cardiovascular risk fell, whereas the usual care group’s rose (−0.57 vs. 0.23). Conclusion: A theory-based telephone-delivered follow-up intervention utilizing motivational interviewing techniques and focusing on personalized action planning demonstrated improvements in patients’ level of activity and health status 12-months postrehabilitation discharge and may be a beneficial supplement to rehabilitation programs.