When you add phone verification or 2FA to your product, you are buying more than “send a code.” You are buying a user experience that shows up at the most fragile moment in your funnel.
Signup, login, password reset, address changes, payout changes, driver handoffs, and account recovery all sit on the same thin line. If the code does not arrive, your user blames your product, not the mobile network.
A Verify API is the layer that turns one-time passwords into a managed workflow. It gives you rules, fallback options, audit trails, and cost visibility. It also gives your teams shared language.
Marketing wants fewer drop-offs. Product crew wants fewer support tickets. Security wants fewer takeovers. Procurement wants predictable pricing. A well-built Verify API can support all of that, but only if you know what you should expect before you integrate it.
This guide walks you through:
- The workings of a modern Verify API
- What can go wrong in real-time traffic
- How you can evaluate a provider in a way that matches business messaging reality.
What a Verify API Actually Does
A Verify API sits on top of a CPaaS and an A2P messaging platform. A2P means ‘application-to-person’ messaging, where your system triggers messages to end users for transactional purposes like codes, alerts, and confirmations. A CPaaS (Communications Platform as a Service) typically bundles the messaging rails, reporting, and developer tooling.
If you send OTPs using a basic SMS API, you usually handle these tasks yourself:
- generate a code
- store it securely
- set an expiry time
- send the message
- rate-limit resends
- verify the user input
- log outcomes
- route delivery errors and retries
A Verify API takes over much of that orchestration. It can still let you bring your own codes, but the key value is the workflow management around delivery and verification.
SMS.to’s Verify API, for example, is built around a single “Verify App” as the source of truth, per-request overrides, multi-channel fallback, and delivery plus verification telemetry through logs and webhooks.
The Threat Landscape OTP Flows Face Today
Most teams adopt verification after something painful happens. A wave of fake signups. A bot farm that hammers password reset. A few account takeovers that turn into refunds, chargebacks, or reputational damage.
The FTC reported consumers lost more than $12.5 billion to fraud in 2024, up 25% year-over-year, and that the share of people who reported losing money jumped from 27% to 38%.
Verification is not the only control you need, but it is a widely used friction point that reduces risk for high-value actions and helps you separate real users from automated abuse.
You also need to treat verification as a security product, not a marketing checkbox. Attackers target OTP flows directly. Microsoft’s security team has stated that multi-factor authentication can block over 99.9% of account compromise attacks in their observed data.
That claim does not mean SMS-based OTP is perfect. It means adding a second factor changes the attacker’s cost. Your job is to choose a Verify API that delivers that benefit without damaging conversion.
Checks to Set Internally
A Verify API is not only about whether messages can be sent. It is about whether real users can finish the task they came to do.
Before you talk to any vendor, align your stakeholders around measurable outcomes. You do not need perfection on day one, but you do need a scoreboard.
What you should always measure
Delivery speed
- median time to first delivery in your top markets
- 90th percentile time to first delivery for OTP flows
Completion
- verification completion rate
- resend rate
- drop-off rate during verification
Cost
- average cost per successful verification
- share of verifications that require fallback
- cost variance by country
Operations
- support tickets per 10,000 verification attempts
- time to resolve a deliverability incident
- webhook failure rate
If you are a marketer or comms lead, this matters because a broken verification step can look like a campaign failure, even when your top-of-funnel is healthy. If you are a product owner, your funnel analytics will lie to you unless you track verification outcomes explicitly. If you are in logistics, verification failures can create real-world delays when drivers, dispatchers, or customers cannot confirm a change quickly.
Coverage, Order, and Fallback Rules for Multi-Channel Delivery
You will hear a lot of talk about “global delivery.” In practice, user behaviour differs by region and by context. A code for a password reset at 2 a.m. may get ignored even if it arrives, while a delivery reschedule code may get acted on immediately.
A modern Verify API should support multiple delivery channels and let you control the order and timing. SMS.to’s Verify API handlesOTP delivery across SMS, Viber, WhatsApp, and Telegram, with automatic fallback until delivered or timed out.
What good fallback looks like in real flows
You want a system that can do all of this without you building complex logic in your app:
- try your preferred channel first
- wait a defined time window
- switch to the next channel if delivery fails or times out
- stop after one successful delivery
- keep the user experience clean so they do not receive multiple active codes
An example from ecommerce:
- A customer checks out, then gets prompted to verify a phone number before high-value delivery
- Your Verify API tries a rich channel first for users who rely on messaging apps
- If the user does not have that app available, fallback sends an SMS
- The user receives one usable code and completes verification without needing support
An example from logistics
- A customer tries to change the delivery address for a parcel
- You treat this as a higher-risk action because it changes where goods go
- Your Verify API sends a code with a short expiry time and a clear message template
- If SMS delivery is delayed, fallback switches channels fast enough to keep the workflow moving
What you should ask for during evaluation
- a country coverage view by channel
- the exact fallback triggers
- what happens when a channel is “unavailable” versus “not delivered yet”
- whether fallback is event-driven or purely timer-driven
- whether you can override rules per request, such as for high-risk actions
SMS.to’s Verify API supports per-request overrides for channel order, expiry, templates, and locale, which is the kind of control you want once you have more than one verification use case.
Pricing and Billing Factor
OTP flows can generate large volumes quickly. A pricing model that is hard to forecast creates constant friction between product and finance.
You should expect two layers of cost:
- the channel delivery fee
- the verification orchestration fee
What matters is how those costs behave when fallback happens. SMS.to publishes clear charging rules for Verify API:
- if WhatsApp, Viber, or Telegram delivers the code, you pay only that channel’s fee and the Verify fee
- if it falls back to SMS, SMS is charged on send, and the Verify fee is charged only if delivered
- no charges for skipped channels or channels that tried but did not deliver
- one channel fee per OTP, and the Verify fee only on successful delivery
That kind of clarity is what you should expect from any provider you shortlist.
A forecasting example you can use internally
Assume you run 100,000 verification attempts in a month.
- 70% delivered on the first channel
- 20% require fallback
- 10% fail after all attempts
In a clean model, your finance team should be able to estimate:
- the delivery fees tied to successful channel deliveries
- the verification fees tied to successful delivery
- the cost exposure during an abuse event, based on your caps and rate limits
If a vendor cannot walk you through that math with your numbers, you will spend months explaining invoices later.
Webhooks and Logs Are The Visibility Layer You Need
A Verify API should give you visibility that matches how you run your product. This is where a lot of “SMS OTP” implementations fall apart. If you cannot trace what happened, your support team ends up guessing, and your users pay for it.
What you should see in the data
At minimum, you want:
- a request identifier you can store in your user session
- an event stream of delivery attempts per channel
- a clear verification outcome such as verified, expired, failed, cancelled
- timestamps for send, delivery, and verification
- enough metadata to segment by country, channel, and template
What your engineering team will build around it
Even with a great Verify API, your webhook handler needs production hygiene:
- accept events fast
- verify authenticity
- process idempotently so duplicates do not break state
- persist a timeline for audit and support
This matters to marketers too. If you run campaigns and see a conversion dip, you want to separate “bad traffic” from “verification friction.” Logs and webhooks are the only honest way to do that.
Security Controls and Abuse Resistance That Protect OTP Endpoints
Verification endpoints are a favourite target for bots because they are cheap to attack and expensive to defend if you have no controls.
A good Verify API should help you with:
- rate limits per phone number and IP
- resend cooldowns
- attempt caps per time window
- configurable expiry times
- protection against verification brute force
You also need to understand the limits of phone-based delivery for out-of-band authentication. NIST classifies the use of the public switched telephone network (PSTN) for out-of-band secrets, including SMS and voice, as restricted, and recommends that verifiers consider risk indicators such as device swap, SIM change, and number porting before using PSTN delivery.
That guidance is not anti-SMS. It is a reminder that phone numbers can be reassigned or hijacked and that your product should treat number changes as higher risk.
SIM swap risks vary by market, but the operational reality is consistent: They happen often for legitimate reasons, and fraudsters try to abuse that process. A Verify API does not solve SIM swap fraud on its own. What it can do is help you implement safer behaviour:
- add extra friction when a phone number was recently changed
- shorten expiry windows for higher-risk actions
- require additional checks when you detect abnormal patterns
- move delivery to channels less exposed to SIM swap when appropriate
Message Templates and Localization For Your OTPs
Business messaging lives or dies on clarity. OTP content is not a place for creative writing. You want the user to understand three things instantly:
- this is your product
- this is the code
- this is the time window
- this is what to do if they did not request it
You should expect template controls such as:
- variable fields for product name and code
- locale selection
- per-market template variants
- short expiry messaging
- optional client-generated OTP support if your security model requires it
SMS.to’s Verify API comes with localization with pre-translated templates for multiple languages and also supports client-generated OTP when you need full control.
Message length drives cost and delivery
Many teams learn this the hard way. Encoding changes can shrink how much text fits in one SMS segment.
User data header reduces the available space. For GSM 7-bit concatenated messages, the max per segment is 153 characters, and for UCS2 concatenated messages it can be 67 characters.
If your verification message contains special characters, emojis, or non-Latin scripts, you can shift into UCS2 more often. That can increase message parts and cost, and it can affect delivery timing. Your Verify API should help you control templates and keep OTP messages lean.
Also Read: SMS pricing and character limits explained for business users
Reliability at Scale, for Performance Under Pressure
Even if you are not sending huge volumes today, you should assume growth. Verification traffic often spikes during:
- new feature launches
- paid campaigns
- seasonal sales
- incident recovery after an outage
- fraud attacks and bot waves
The global messaging environment is also large enough that telecom economics matter. A MobileSquared market report estimates total A2P SMS traffic peaked in 2024 at 2.84 trillion messages and forecasts it declining to 2.11 trillion by 2029. It also highlights smishing traffic volumes on white routes and the role of filtering and enforcement in cleaning up the ecosystem.
That scale is one reason you should expect your Verify API provider to have mature routing, monitoring, and anti-abuse practices.
On reach, the ITU estimates 9.2 billion mobile-cellular subscriptions in 2025, which equals 112 subscriptions per 100 inhabitants, and notes variation by income group and region.
You are building a verification workflow that can touch billions of endpoints, but your user experience still feels personal. Reliability is non-negotiable.
What Each Team Gains From a Strong Verify API
Different teams care about different wins. A strong Verify API supports all of them. For instance:
Marketers and comms teams
- Fewer signups lost to “code never arrived”
- Fewer social posts and support escalations about login issues
- Better trust messaging when fraud spikes happen
- Clearer analytics that separates traffic quality from delivery problems
Product owners
- One configuration surface for multiple verification flows
- Ability to tune expiry and channel order by use case
- Telemetry that supports conversion work
- Lower support burden and fewer manual account recoveries
Logistics operators
- Verification that keeps operational workflows moving
- Step-up checks for address changes, pickup release, and driver onboarding
- Fewer customer calls triggered by delivery friction
- Audit trails for disputes and compliance
Ecommerce teams
- Reduced fraud at checkout and account recovery
- Better completion in markets where SMS delivery can be inconsistent
- Clearer cost per successful verification to support unit economics
Run This Evaluation Checklist
You do not need months of pilots to learn a lot. You need targeted tests that match your business. And you can do it in a week.
Define your top markets and run real device tests
Pick at least five markets that represent your user base. Include at least one market where you know delivery can be slower.
Here you want to look at:
- time to first delivery
- resend experience
- fallback behaviour
- completion rate
Test a high-risk workflow
Choose one action that matters to fraud and user trust.
Examples:
- change password and change email
- change delivery address
- add payout method
- approve a high-value order
Shorten expiry and tighten resend rules. Check whether your Verify API supports per-request overrides so you can do this without creating a separate integration. SMS.to’s Verify API supports per-request overrides for channel order, expiry, templates, and locale, which is the type of capability you should look for.
Simulate abuse
You do not need to run a real attack. You can simulate abnormal patterns:
- repeated OTP requests for the same number
- bursts from the same IP range
- rapid-fire resends
Your evaluation criteria:
- rate limits trigger predictably
- legitimate users can still verify
- logs show what happened clearly
- cost controls stop runaway spend
Validate billing logic with a sample invoice
Ask for a sample breakdown that matches your test traffic:
- delivered on primary channel
- delivered after fallback
- failed deliveries
- expired codes
If the provider publishes charging rules, compare them to the invoice behaviour.
With SMS.to, Verify API pricing stays predictable: you pay one channel fee for the channel that actually delivers the OTP, plus the Verify Fee only when delivery succeeds—and you’re not charged for skipped channels or failed delivery attempts. For example, if WhatsApp, Viber, or Telegram delivers the code, you’re charged only that channel’s fee plus the Verify Fee; if it falls back to SMS, SMS is charged on send and the Verify Fee applies only if the SMS is delivered.
OTP Verification Done Right
A Verify API is part security control, part conversion tool, and part messaging infrastructure. You should expect it to give you:
- multi-channel reach with controlled fallback
- transparent charging that matches real delivery outcomes
- telemetry you can use during incidents
- rate limits and resend controls that reduce abuse
- template and localization controls that keep messages clear
- the flexibility to tighten verification for high-risk actions
If you treat verification as a core product surface and evaluate it with real workflows, you end up with fewer drop-offs, fewer account takeovers, and fewer internal debates about what “really happened” to an OTP.
