The cheapest way to verify an Indian phone number is not the cheapest SMS route. It is sending over WhatsApp first at ₹0.115 and falling back to SMS only for the users WhatsApp could not reach — which costs about a third less than SMS-only without giving up anyone.
Why WhatsApp-first is cheaper and also better
Usually those two pull against each other. Here they do not, for three reasons:
- The rate. ₹0.115 per delivered authentication template against ₹0.15–₹0.25 for an Indian transactional SMS.
- No DLT on the WhatsApp leg. No entity registration, no sender ID, no per-template approval with an operator. DLT still applies to the SMS fallback — see the DLT guide — but it stops being on the critical path for most of your volume.
- No frequency cap. Authentication templates are exempt from the per-user limits that throttle marketing messages, so a user who logs in five times in a day receives five codes.
The fallback is the whole design
WhatsApp-first only works if the miss is handled. Not every user has WhatsApp, and roughly one in twenty accepted messages never reaches a handset — an unreachable device, a policy block, a stale number.
So the ladder matters more than the rate: send over WhatsApp, wait for a delivery receipt rather than an acceptance, and fall back to SMS the moment it is clear the message did not land. Done properly the user never sees the difference; done badly they simply cannot log in. The fallback mechanics are worth reading before you build it yourself.
What "cheapest" usually hides
A low advertised rate is easy. Two things quietly undo it:
- Being charged on acceptance rather than delivery. The few percent that never arrive are billed anyway, and the statement cannot distinguish them from messages that did.
- Messages sent under the wrong category. Marketing costs several times utility or authentication for content that is neither.
Both are covered in more detail in what an OTP actually costs in India. They are worth more than the difference between two rate cards.
What to hold a provider to
- One API that routes across WhatsApp, SMS and voice — not three integrations and three invoices.
- Automatic fallback on non-delivery, not on non-acceptance.
- DLT registration handled during onboarding.
- Auto-read SDKs — Android SMS Retriever and iOS one-time-code — so nobody types the digits.
- Billing on confirmed delivery, in INR, on Indian routes.
- SMS-pumping protection at the request step, before inflated traffic costs you.
QuickAuth is built around that list: a Direct Meta Tech Provider for WhatsApp with direct operator routes for SMS, one API, one INR invoice, and charges made final only when a message is confirmed delivered.