Objective: To investigate whether simultaneous telephone coaching improves the clinical effectiveness of a physiotherapist-prescribed home-based physical activity program for knee osteoarthritis (OA). Methods: A total of 168 inactive adults ages ≥50 years with knee pain on a numeric rating scale ≥4 (NRS; range 0–10) and knee OA were recruited from the community and randomly assigned to a physiotherapy (PT) and coaching group (n = 84) or PT-only (n = 84) group. All participants received five 30-minute consultations with a physiotherapist over 6 months for education, home exercise, and physical activity advice. PT+coaching participants also received 6–12 telephone coaching sessions by clinicians trained in behavioral-change support for exercise and physical activity. Primary outcomes were pain (NRS) and physical function (Western Ontario and McMaster Universities Osteoarthritis Index [WOMAC; score range 0–68]) at 6 months. Secondary outcomes were these same measures at 12 and 18 months, as well as physical activity, exercise adherence, other pain and function measures, and quality of life. Analyses were intent-to-treat with multiple imputation for missing data. Results: A total of 142 (85%), 136 (81%), and 128 (76%) participants completed 6-, 12-, and 18-month measurements, respectively. The change in NRS pain (mean difference 0.4 unit [95% confidence interval (95% CI) −0.4, 1.3]) and in WOMAC function (1.8 [95% CI −1.9, 5.5]) did not differ between groups at 6 months, with both groups showing clinically relevant improvements. Some secondary outcomes related to physical activity and exercise behavior favored PT+coaching at 6 months but generally not at 12 or 18 months. There were no between-group differences in most other outcomes. Conclusion: The addition of simultaneous telephone coaching did not augment the pain and function benefits of a physiotherapist-prescribed home-based physical activity program.