Direct smartphone-to-satellite connectivity is currently under investigation in both 5G Advanced and 6G to extend the support of basic communication services to the most remote geographical areas. While current satellite connectivity systems are focused on offering a global service for emergency and text messaging, their support for other extended data services, such as multimedia services, remains limited due to the challenging wireless conditions of non-terrestrial networks (NTN). These limitations stem from the significantly longer propagation distances between the device and the satellite resulting in poor link budgets and high latencies. To address these challenges and improve the reliability of data transmission in satellite networks, leveraging repeated transmissions of transport blocks with both low-rate modulation schemes and incremental redundancy can be key. In this paper, the focus is on voice services. Particularly the problem of optimally selecting the modulation coding scheme and the number of scheduled repetitions to maximize the probability of successful voice packet reception within a fixed end-to-end latency. The feasibility of supporting voice calls over 5G-NTN under different practical conditions is then evaluated for the optimized scheduler. Link-level simulations are performed based on a 3GPP 5G-NTN model, integrating 5G NR waveforms, LDPC coding, and HARQ with incremental redundancy. Numerical results suggest that high-quality voice calls can be supported in 5G-NTN for a wide range of altitudes and elevation angles, even at 2,000 km satellite altitude, which is the highest altitude considered for LEO satellites. Moreover, numerical results indicate that HARQ retransmissions are unnecessary for supporting voice transmission, as proactively scheduling a large number of repetitions is sufficient. However, using both scheduled repetitions and HARQ retransmissions is more efficient in terms of radio resources and power consumption.