Information and Communication Technology interventions have the potential to improve outcomes in health and other development sectors in low-income settings. Large-scale impact, however, remains the central challenge for the HCI4D community as significant and diverse resources are typically required to scale such interventions beyond the pilot stage. In contrast, voice-based entertainment services accessible over simple phones, designed for similarly low-income, low-literate populations manage to scale 'virally' to tens of thousands of users with little to no advertising cost. Our study compares the outcomes of using voice-based entertainment to spread a maternal-health hotline against conventional advertisement channels including paper flyers, posters, radio, TV, social media and robocalls. Through an 11-week deployment in Pakistan where the hotline reached 21,770 users over 32,625 calls, we find that the entertainment service outperformed other channels on all popular user acquisition metrics, with the exception of robocalls, which lead in terms of spread.