Purpose Transfer from on-site rehabilitation to the participant's daily environment is considered a weak link in the rehabilitation chain. The main objective of this study is to see if adding boosted telephone follow-up directly after completing an occupational rehabilitation program effects work participation. Methods A randomized controlled study included participants with chronic pain, chronic fatigue or common mental disorders on long-term sick leave. After completing 3½ weeks of acceptance and commitment therapy based occupational rehabilitation, participants were randomized to boosted follow-up or a control group before returning to their daily environment. The intervention was delivered over 6 months by on-site RTW coordinators mainly via telephone. Primary outcome was RTW categorized as participation in competitive work ≥1 day per week on average over 8 weeks. Results There were 213 participants of mean age 42 years old. Main diagnoses of sick leave certification were mental disorders (38%) and musculoskeletal disorders (30%). One year after discharge the intervention group had 87% increased odds (OR 1.87, 95% confidence interval 1.06–3.31, p = 0.031), of (re)entry to competitive work ≥1 day per week compared with the controls, with similar positive results for sensitivity analysis of participation half time (≥2.5 days per week). The cost of boosted follow-up was 390.5 EUR per participant. Conclusion Participants receiving boosted RTW follow-up had higher (re)entry to competitive work ≥1 day per week at 1 year when compared to the control group. Adding low-cost boosted follow-up by telephone after completing an occupational rehabilitation program augmented the effect on return-to-work.