SMS remains the most reliable channel for sending one-time passcodes (OTP), despite the growth of other alternatives. It’s simple, doesn’t require the user to have an app installed, and works even with limited data connectivity. But not all SMS providers offer the same level of control or service. Here are the points worth reviewing before choosing one.
This guide focuses only on sending OTP codes. If you need to evaluate a provider for bulk, transactional or marketing SMS, see How to choose an SMS provider in Mexico: 12 questions to ask before signing. To understand the mechanism, see SMS OTP authentication: how it works.
1. Code delivery speed
A code that expires in five minutes and arrives in three leaves the user no margin. Ask what the typical OTP delivery time is, how it behaves at peak times (for example, during El Buen Fin) and whether code traffic is processed with priority over bulk campaigns, so a promotional send never delays an authentication.
2. Verification of the number against the National Numbering Plan
A serious provider validates every number against Mexico’s National Numbering Plan (PNN) before sending, to know whether it is a landline or mobile. In an OTP flow this matters twice: you avoid paying for codes that were never going to arrive, and you can offer the user another option, such as a voice call, when they registered a landline.
3. Encoding and code template
The code message should fit in a single segment. The encoding (GSM7 or UCS-2) defines how many characters fit, and one accent or emoji can split the SMS in two and double the cost. A good provider lets the client choose the encoding and automatically replaces characters not supported by GSM7. Check the template too: what the code is for, how long it lasts and the warning not to share it. We explain it in how many characters an SMS has.
4. Voice call fallback
There will always be users the SMS does not reach: a suspended line, weak coverage or a landline. NIST, the most cited technical reference on authentication, requires an alternative method to be available to every user. Ask whether the provider can deliver the same code by voice call as an automatic fallback and how that retry is configured.
5. Protection against abuse and SMS pumping
Forms that trigger an OTP are a target for bots that generate thousands of requests to numbers controlled by the attacker (SMS pumping). Ask which controls the provider offers: sending limits per number and per IP, alerts on abnormal traffic spikes and blocking of suspicious destinations. On your application side, limit retries and failed attempts, as the NIST guidelines recommend.
6. Recognizable sender and Flash SMS
An alphanumeric sender (mask) makes the code arrive with the brand name instead of a random number, which helps users tell it apart from a smishing attempt. For codes that must be seen immediately, a Flash SMS appears directly on the phone screen without opening the messaging app.
7. Real-time delivery reports and specialized support
For an OTP it is not enough to know the message was sent: you need the delivery report (DLR) for every code in real time, ideally by webhook, to spot an outage before it turns into locked-out customers. And when something fails in production, every minute counts: confirm whether there is priority support, a real person on the other end and the ability to build custom solutions when your case does not fit the standard mold.
At Grupo Tecnophone we built our infrastructure with these seven points in mind, with specialized support and custom development on an architecture designed for critical messaging. If you’d like to try it with your own use case, we have a 30-day pilot available.
Sources
- “Digital Identity Guidelines: Authentication and Authenticator Management” (SP 800-63B-4), NIST. Read source
- “CNBV abre la puerta a que bancos envíen códigos de seguridad por SMS y simplifica consultas de saldo”, El CEO, September 2026. Read article
