An order placed with an unreachable number was never going to be delivered.
The largest single category of COD failure is orders placed with a number that was never reachable, and OTP addresses it directly.
Conversion will fall slightly, and that is the mechanism working — judge on delivered revenue, never on conversion rate.
Delivery speed is the implementation detail that decides whether genuine buyers abandon at the step.
WhatsApp usually outperforms SMS on speed and open rate in markets where it dominates messaging.
The shopper enters their phone number, receives a one-time code by SMS or WhatsApp, and enters it before the order is placed.
The full guide has the formats, the deadlines and the worked examples.