The SMS OTP is the most widely used second authentication factor in the world. Here is how it works, which design decisions matter and how to implement it with an SMS API.
What an OTP is and why SMS is still the most used channel
OTP stands for One-Time Password: a single-use code, usually 4 to 8 digits, valid for one operation and expiring within a few minutes. It is used as a second authentication factor (2FA) because it proves the person has access to a specific device, the phone, on top of knowing their password.
Other mechanisms exist, such as authenticator apps or push notifications, but SMS remains the most widespread for one simple reason: it works on any handset, with nothing to install and no internet connection. For a bank, a fintech or an online store serving millions of people with very different devices, that universality outweighs every other advantage.
How the flow works, step by step
The whole process takes a few seconds and runs through six stages. First, the user performs an action that requires verification: creating an account, signing in from a new device, authorising a transfer. Second, the company backend generates a random code, ties it to the user and the operation, and stores it with an expiry time (normally between 2 and 10 minutes).
Third, the backend calls the SMS provider API with the user number and the message text. Fourth, the platform validates the request, routes it to the relevant mobile operator and the operator delivers the SMS to the handset. Fifth, the user reads the code and types it into the application. Sixth, the backend compares the entered code with the stored one, checks that it has not expired and that the allowed number of attempts has not been exceeded, and approves or rejects the operation.
One important point: the code is always generated and validated by your own system, never by the SMS provider. The messaging platform is responsible for delivering the message quickly and telling you whether it arrived.
Design decisions that make the difference
- Length and format: six numeric digits is the standard. Longer codes are harder to type; shorter ones reduce security.
- Expiry: 2 to 5 minutes for sensitive operations; up to 10 for sign-ups. A code that lives too long is an open door.
- Attempts: cap at 3 to 5 attempts per code and apply a temporary block after several consecutive failures.
- Resend: allow a resend only after 30 to 60 seconds and with a daily cap, to prevent abuse and unnecessary cost.
- Message copy: include the company name, the code and a clear warning not to share it. Avoid links.
- One code per operation: never reuse the same OTP for different actions.
One further recommendation is to store the identifier (sid) the API returns for each message, so you can match it against the delivery report if a user reports never receiving the code.
Security: threats and how to mitigate them
The best known risk is SIM swapping, where an attacker persuades the operator to port the victim number to another SIM card. It is mitigated by combining the OTP with other signals (known device, usual location, amount limits) and by offering an alternative second factor for high-risk operations.
The second risk is social engineering: someone impersonates the company and asks the customer to read out the code. That is why the SMS text must state explicitly that nobody from the company will ever ask for it. The third is abuse of the send endpoint (SMS pumping), controlled with rate limits per number and per IP, and by validating number format before sending.
On the platform side, Grupo Tecnophone adds its own layers: token authentication, an allowed-IP list per connection and optional RSA-OAEP encryption of the number and the message body, so sensitive data travels protected even inside the HTTPS channel.
How to implement it with the Grupo Tecnophone SMS API
The integration comes down to one HTTPS call per code. In the platform console you create an API connection, copy the token and register the IP addresses your backend will call from. After that, sending is a POST request to the send endpoint with two fields: the destination number and the text.
curl -X POST https://api.grupotecnophone.com/prod/v1/sms/send \
-H "Authorization: Bearer YOUR_TOKEN" \
-H "User-Agent: MyApp/1.0" \
-H "Content-Type: application/json" \
-d '{"to": "+5215512345678", "body": "Your MyApp code is 482913. It expires in 5 minutes. Do not share it."}'The response includes a unique message identifier (sid), the initial status, the encoding used and the number of segments. A sandbox environment validates the whole logic without sending real messages, which is ideal for testing the integration before going live. Full documentation, with examples in PHP, Python, Node and Java, is in the Developers section.
What to measure after going live
Three indicators tell you whether your OTP works: delivery rate (messages delivered over messages sent), latency (seconds between the request and delivery) and the conversion rate of the flow (users who complete verification). If delivery is high but conversion is low, the problem is usually the message copy or the experience of the code entry screen.
Per-message and per-operator delivery reports let you quickly tell whether an issue is general or concentrated in one operator or region.
Frequently asked questions
Need an enterprise SMS platform?
Grupo Tecnophone provides enterprise SMS messaging with direct routes in Mexico, an API with sandbox and encryption, per-message delivery reports and a console for campaigns. Talk to an expert or read the developer documentation.