Every product team hits this decision. You need to send a login code, an order update, a payment alert. SMS is the default because it has always been the default. But defaults deserve a second look, so here is the honest version, including where SMS still beats WhatsApp.
Deliverability: the quiet killer
The worst thing about SMS is that it fails silently. Your gateway reports "sent," the message hits a chain of carriers and aggregators, and somewhere in there a chunk of them just never arrive. You do not get told. The first you hear of it is a support ticket: "I never got my code."
WhatsApp runs over data, straight to an app the person already has open. It is not magic and it is not 100 percent, but you get real delivery and read signals back, so you actually know what happened instead of guessing.
If your OTP delivery is a mystery today, that alone is a reason to look.
Cost: it depends, and you should do your own math
Do not trust anyone who tells you one is always cheaper, including me. It depends on your country, your volume, and your provider. In a lot of African markets, though, SMS is priced per message with no relationship to whether it landed, so you can end up paying for messages that never arrived. WhatsApp pricing works differently and is often lower for the same reach.
The point is not "WhatsApp is cheap." The point is: add up what you currently pay for messages that fail, then compare. Most teams have never done that calculation.
User experience
A code in an SMS is a code in an SMS. A message on WhatsApp can carry your business name, a bit of context, a link, even an image of the receipt. To the customer it feels like they are dealing with a real business, not a random shortcode. That matters more than people admit, especially for trust-sensitive things like payments.
Where SMS still wins, honestly
This is not a WhatsApp-solves-everything post. SMS is genuinely better when:
Your users do not all have WhatsApp or a smartphone. Feature phones and no-data situations are real, and SMS reaches them. WhatsApp cannot.
You are in a regulated flow that mandates SMS, which some banking and government processes still do.
You need it to work with zero internet on the receiving end. SMS rides the cell network, WhatsApp needs data.
For a lot of products the honest answer is not "switch" but "add WhatsApp as the primary channel and keep SMS as the fallback." Best of both.
How to actually decide
Ask three questions about your own users:
Do most of them have WhatsApp? For consumer apps in much of Africa and beyond, yes.
Are you paying for SMS that silently fails? Pull your logs and your support tickets and find out.
Would your messages be better with the business name, context and a link attached? Usually yes.
If two of those point to WhatsApp, run a small test. Send the same OTP or alert through both for a week and compare delivery, complaints and cost. Let the numbers decide, not the default.
Bottom line
SMS is the habit. WhatsApp is often the better channel for reach, feedback and cost, but not always, and pretending otherwise would be dishonest. Do the comparison on your own traffic. The teams that actually measure it are usually surprised by how much they were losing to messages that never arrived.