Duolingo partnered with Twilio to test using WhatsApp as a primary verification channel with SMS as a fallback for One-time Passcodes in select countries, resulting in improved OTP conversion rates and lower costs per OTP, with some markets showing higher success rates and reduced costs when sending via WhatsApp instead of SMS. The experiment showed that using WhatsApp as an OTP delivery channel can be more cost-effective and convert users better than traditional SMS-only methods, but also comes with regional nuances and varying customer preferences.