IMPORTANCE There is limited information as to whether telephone or short message service (SMS) support is effective in improving infant feeding practices and tummy time and reducing screen time. OBJECTIVE To determine the effectiveness of either nurse-led telephone or SMS support in improving infant feeding practices and tummy time and reducing screen time. DESIGN, SETTING, AND PARTICIPANTS This study was part of a 2-year, 3-group parallel, randomized clinical trial conducted from February 23, 2017, to November 30, 2018, among 1155 women in the third trimester of pregnancy in New South Wales, Australia. It reports the main outcomes at 6 and 12 months of child age. All analyses were conducted on an intention-to-treat principle. INTERVENTIONS The intervention consisted of staged information booklets mailed to the intervention groups, each followed by either a nurse-led telephone support session or SMS intervention, antenatally and at 1, 3, 5, 7, and 10 months after birth. MAIN OUTCOMES AND MEASURES The primary outcomes were infant feeding practices at both 6 and 12 months and tummy time at 6 months. The secondary outcome was screen time at 12 months. RESULTS Of 1155 mothers, 947 (82%; mean [SD] age, 32.5 [5.0] years) completed follow-up surveys at 6 months; 920 mothers (80%) completed follow-up surveys at 12 months. Compared with the control group, telephone support led to higher odds of appropriate timing of introducing solid foods (adjusted odds ratio [AOR], 1.68 [95% CI, 1.22-2.32]), cup use (AOR, 1.54 [95% CI, 1.12-2.13]), and early-start tummy time (AOR, 1.63 [95% CI, 1.18-2.25]) at 6 months and higher odds of having no screen time (AOR, 1.80 [95% CI, 1.28-2.53]) and no bottle at bedtime (AOR, 1.73 [95% CI, 1.23-2.42]) at 12 months. Use of SMS also led to higher odds than the control group of having no screen time (AOR, 1.28 [95% CI, 1.08-1.52]) and having no bottle at bedtime (AOR, 1.29 [95% CI, 1.10-1.51]) at 12 months. No significant differences were found in breastfeeding rates between the telephone support, SMS support, and control groups. CONCLUSIONS AND RELEVANCE Both the nurse-led telephone support and SMS interventions were effective in reducing screen time and bottle use at bedtime. Telephone support was also effective in promoting the appropriate timing of the introduction of solid foods, early-start tummy time, and cup use.