Why SMS OTPs are becoming a liability
SMS has long been the default channel for verifying a login or a checkout in India, and while that has mostly served us well, the ability to deliver transactional messages has seen a steady decline in recent years due to telecom filtering, DLT registration hassles for businesses and reduced open rates as users prefer apps to SMS. None of these factors are outright deal killers, but taken together they significantly impact the reliability of SMS based verification as compared to what was previously possible.
How WhatsApp OTP compares
WhatsApp, as a messaging channel, has much better open and read rates compared to traditional SMS, because users actively engage with WhatsApp more often and with higher frequency. Combined with near-instant delivery as a result of being on the same platform as the receiver, WhatsApp OTPs are a much more reliable mechanism for verification, with much lower drop-off at the verification step as compared to SMS.
How it works
WhatsApp OTP messages are sent using the authentication message category. This category has a predefined format (unlike marketing or utility messages) for WhatsApp messages, which is essentially a short code style verification code. Implementation-wise, this means adding a WhatsApp OTP step (alongside or instead of an SMS step) to your verification flow, at the point where the user is currently being asked to enter a verification code. The entire process should be comparable to implementing any other type of WhatsApp message.
What about fallbacks?
While WhatsApp messages have a very high delivery/read rate, they are not 100% reliable (due to both network issues and users blocking WhatsApp numbers) and for critical flows, it is advisable to implement a fallback option like SMS for those who fail to receive a WhatsApp message. The implementation for such a fallback would depend on your specific use-case, but as a rule of thumb, you should attempt WhatsApp messages first (due to higher reliability) and resort to SMS for the (small) subset of users who were unable to receive WhatsApp messages.
Next steps
The actual implementation of WhatsApp OTP within your application is a relatively straightforward process, as the verification process itself is not materially different from what you are likely already using for SMS. As such, your team should be able to find the relevant WhatsApp documentation pertaining to your authentication method (marketing, utility, or authentication message) and build upon that.