Adding SMS verification to an application can seem straightforward: generate a one-time password, send it to the user’s phone and verify the code they enter. In practice, a reliable OTP API integration needs to account for much more, including API authentication, message delivery, retries, code expiry, error handling and delivery status.
For developers building applications for Singapore users, there are also additional considerations around SMS routing, Sender IDs and deliverability. An SMS verification API connects your application to the messaging infrastructure, allowing your system to trigger verification messages programmatically whenever a user needs to be authenticated.
This guide walks through the typical integration process for developers in Singapore, from making your first API request and sending SMS via HTTP to handling delivery receipts and building a more reliable OTP verification flow.
How Does an SMS Verification API Work?
An SMS verification API allows your application to trigger SMS messages programmatically through an HTTP-based API. Instead of sending messages manually, your application makes an API request containing information such as the recipient’s phone number and message content.
For an OTP flow, the process typically looks like:
User requests verification → Application generates OTP → API sends SMS → User enters OTP → Application validates OTP → User is authenticated
The SMS API handles the message delivery layer, connecting your application to the messaging infrastructure. Your application remains responsible for the authentication logic around the OTP.
This separation is important. A successful SMS delivery does not mean that an OTP has been securely verified. Your application still needs to handle controls such as:
- OTP generation
- Code expiry
- OTP validation
- Attempt limits
- Rate limiting
- Error handling
In other words, the API takes care of getting the message to the user, while your application determines whether the code is valid and whether the user should be authenticated.
What Do You Need Before Integrating an SMS API?
Before you start your SMS API integration, make sure you have the following in place:
- An account with an SMS API provider – This gives your application access to the provider’s messaging infrastructure and API services.
- API credentials or authentication keys – These authenticate requests from your application and help prevent unauthorised access or use of the API.
- A sending identity, where required – Your Sender ID or originating number identifies your business to recipients. Depending on the destination and Sender ID you use, local registration requirements may apply.
- The recipient’s phone number – Store and format phone numbers correctly, including the appropriate country code for Singapore numbers (+65).
- SMS content or an OTP template – Keep verification messages clear and concise, and make sure they meet any applicable messaging requirements.
- A backend environment for API requests – API credentials and OTP verification logic should generally be handled server-side rather than exposed in your client application.
- API response handling – Your application needs to process responses from the SMS provider and distinguish between accepted requests, rejected requests and errors.
- Delivery receipts or webhooks – Where supported, webhooks can send delivery-status updates back to your application so you can track whether messages were delivered or failed.
- OTP generation and validation logic – If you’re using SMS for verification, your application must generate, expire and validate OTPs securely rather than relying on the SMS API alone.
For Singapore deployments, also check whether your Sender ID and messaging route meet applicable local requirements, including relevant SSIR requirements where an alphanumeric Sender ID is used.
You should also check what the provider’s API supports before you start building. Look at the available endpoints, authentication methods, delivery reporting, supported messaging channels and webhook capabilities to make sure they fit your application’s requirements.
Once these pieces are in place, you can move on to making your first HTTP request and sending an SMS through the API.
How Do You Send an SMS via HTTP API?
Most SMS APIs use HTTP requests to send messages. The exact endpoint, authentication method and request parameters depend on the provider.
To send SMS via HTTP, your application makes an authenticated request to the provider’s API endpoint, usually containing the recipient number and message body.
A typical request contains:
POST /messages
Authorization: Bearer YOUR_API_KEY
Content-Type: application/json
{
“to”: “+65XXXXXXXX”,
“text”: “Your verification code is 123456.”
}
The API then returns a response indicating whether the request was accepted.
The example above is illustrative. The exact endpoint, headers, request body and authentication method should always come from your provider’s current API documentation rather than being copied from a generic example.
For 8×8 CPaaS, developers can refer to the 8×8 CPaaS Developer Portal for API references and integration guides covering communication services and products.
How Do You Integrate an OTP Verification API?
An OTP integration involves more than making a single API call. Your application needs to manage the full verification lifecycle, from generating the code and sending the SMS to validating the user’s response and preventing repeated or unauthorised attempts.
A robust OTP API integration should keep SMS delivery separate from your application’s verification logic. This gives you control over how codes are generated, stored, validated and invalidated.
Step 1: Generate the OTP
Generate a random, time-limited code on your server.
For example:
- OTP: 583214
- Validity: 5 minutes
Avoid predictable codes or storing OTPs in plain text where possible. Use appropriate security controls to protect verification data.
Step 2: Store the Verification State
Your application should associate the OTP with the relevant user or verification session.
Depending on your implementation, you may need to store:
- User or session identifier
- OTP or a secure representation of it
- Creation timestamp
- Expiry timestamp
- Number of verification attempts
- Verification status
This gives your application the information it needs to determine whether a submitted code is still valid.
Step 3: Send the OTP Through the API
Your backend sends an authenticated HTTP request to the SMS API containing the destination number and message.
Keep the SMS concise and make the purpose of the code clear. For example:
Your verification code is 583214. It expires in 5 minutes.
Avoid including unnecessary personal, account or other sensitive information in the message.
Step 4: Return a Response to the Application
The SMS API will typically return a response indicating whether the request was accepted, rejected or returned an error.
This is where an important distinction comes in: API acceptance is not the same as SMS delivery.
A successful API response generally means that the provider has accepted the request for processing. It does not necessarily mean that the SMS has already reached the recipient’s device.
Where supported, your application can use delivery receipts or webhooks to receive subsequent updates on message delivery.
Step 5: Ask the User to Enter the OTP
Once the verification SMS has been requested, your application can display the verification screen.
The user enters the code they received, and your application sends the submitted code to your backend for validation.
Step 6: Validate the OTP
Your server should check that:
- The OTP matches the correct verification session.
- The OTP has not expired.
- The OTP has not already been used.
- The user has not exceeded the permitted number of attempts.
If all conditions are satisfied, mark the verification as successful and invalidate the OTP so it cannot be reused.
What Endpoints and Auth Does the SMS API Use?
The exact authentication method and endpoints depend on the SMS provider and the type of API you are integrating.
For a basic SMS API, authentication may use an API key or token passed in the request header. Managed verification services can use a different model, with provider-specific credentials and separate endpoints for starting and checking a verification.
When building an OTP solution for Singapore users, you may encounter two common integration approaches:
| Aspect | Raw SMS Gateway | Managed Verify API |
| Auth method | API key or token passed in the request | Provider-specific credentials and verification service identifiers |
| Code generation | Your application generates and stores the OTP | Provider generates and manages the OTP |
| Endpoints | Typically an SMS send endpoint | Separate endpoints for creating a verification and checking the submitted code |
| Sender ID compliance | You are responsible for applicable Sender ID requirements | Requirements depend on the provider, route and Sender ID used |
| OTP Lifecycle | Your application manages expiry, attempts and invalidation | Provider manages OTP expiry and verification state |
With a raw SMS API, you have more control over OTP generation, validation and security logic, but you also have more responsibility for building and maintaining those controls.
A managed Verify API can handle more of the OTP lifecycle for you, potentially simplifying implementation. However, you still need to understand the provider’s authentication model, API endpoints, rate limits, delivery behaviour and applicable Singapore messaging requirements.
The 8×8 Verification API follows the managed Verify API model. It provides verification through SMS, WhatsApp and voice, with the verification service handling the OTP lifecycle rather than requiring your application to build the code generation and verification logic from scratch. This can simplify implementation when you need to add OTP verification to a sign-up, login, account recovery or transaction flow, such as a banking or e-commerce app.
Always refer to your provider’s current API documentation for the exact authentication headers, endpoint URLs and request parameters. The examples used in one provider’s documentation should not be assumed to work with another SMS API.
How Do I Handle Delivery Receipts and Singapore Routing?
Delivery Receipts (DLRs) are asynchronous status updates that tell your application what happened to an SMS after the API accepted the request. This is different from the initial API response, which only tells you whether the provider accepted the message for processing.
Where supported, your application can receive DLRs through a webhook and use them to track statuses such as delivered, failed or undeliverable.
For Singapore deployments, you also need to consider how your messages are routed and whether your Sender ID meets applicable local requirements.
For example, using an unregistered alphanumeric Sender ID where registration is required can affect whether an SMS is delivered. Depending on the provider and route, the resulting failure may be reflected in delivery-status updates rather than the original API response.
Your application should therefore handle these stages separately:
- API response: Was the message request accepted or rejected?
- DLR: Was the SMS subsequently delivered or did delivery fail?
- OTP verification: Did the user enter the correct, valid code?
How Should You Manage OTP Expiry and Attempts?
Delivery is only one part of a reliable OTP implementation. Your application also needs to control how long an OTP remains valid and how often a user can request or attempt a code.
A practical implementation should:
- Set a short expiry period so unused codes become invalid quickly.
- Invalidate an OTP after successful use so it cannot be reused.
- Limit incorrect attempts to reduce brute-force guessing.
- Apply rate limits to OTP requests, particularly by user, account, device or IP where appropriate.
- Prevent unlimited resend requests that could be used to abuse your SMS service.
- Apply a cooldown period between successive OTP requests where appropriate.
- Generate a new code when required rather than allowing repeated use of an old code.
These controls help protect both the authentication flow and the SMS service itself, reducing the risk of brute-force attempts, OTP abuse and unnecessary message volume.
A simplified verification flow looks like this:
Request OTP → Generate code → Send SMS → User enters code → Check expiry + attempts + match → Verify or reject
If the OTP is invalid, expired or the attempt limit has been reached, the application should reject the verification rather than simply sending the user through the same flow indefinitely.
What Should You Consider for Singapore SMS Routing?
For businesses sending SMS to Singapore users, routing and sender identity can affect message deliverability and the customer experience.
If you’re using an alphanumeric Sender ID, check the requirements of Singapore’s SMS Sender ID Registry (SSIR) and make sure your messaging provider or aggregator supports the applicable requirements.
Your implementation should also account for:
- Singapore mobile number formatting – Use the correct country code and validate numbers before sending.
- Sender ID configuration – Make sure the Sender ID used by your application matches your provider’s configuration and any applicable registration requirements.
- Message routing – Understand how your provider routes SMS to Singapore mobile networks (such as Singtel, StarHub and M1) and whether different routes have different delivery characteristics.
- Delivery reporting – Use DLRs or webhooks where available to monitor delivery outcomes and identify failures.
- Message content and length – Keep OTP messages concise and account for SMS character limits and encoding.
- Filtering and delivery delays – Allow for carrier or network-level filtering and occasional delivery delays when designing the verification flow.
- Applicable local requirements – Make sure your messaging setup aligns with Singapore’s relevant SMS and anti-scam requirements.
These considerations matter particularly for authentication messages, where a delayed or undelivered OTP can prevent a user from completing a login, registration or transaction.
What Are Some Common SMS API Integration Mistakes?
Treating API Success as Delivery
A successful API response usually means the provider accepted the SMS request for processing, not that the message has reached the user’s device.
Where supported, use delivery receipts or webhooks to track what happens after submission
Storing OTPs Insecurely
OTP data should be protected throughout its lifecycle. Avoid exposing verification data unnecessarily, and use appropriate controls for storing, accessing and invalidating OTPs.
Allowing Unlimited Attempts
Without attempt limits, rate limiting and resend controls, attackers may repeatedly guess codes or abuse the SMS service.
Set limits around both verification attempts and OTP requests, rather than relying on code expiry alone.
Sending Sensitive Information in the SMS
An OTP message should contain only what the user needs to complete verification. Avoid including unnecessary personal, account or transaction details that could expose sensitive information if the message is viewed by someone else.
Ignoring Failed Deliveries
If an OTP cannot be delivered, your application should have a clear fallback or recovery path. Don’t leave the user waiting indefinitely or automatically send repeated messages without appropriate controls.
Building OTP Logic Entirely Around SMS
The SMS API handles the messaging layer; your application still needs to manage OTP generation, expiry, validation, attempts and verification state.
Keeping these responsibilities separate makes the integration easier to secure, troubleshoot and adapt as your authentication requirements change.
How Can 8×8 CPaaS Support Developers With SMS and OTP Verification API in Singapore?
8×8 CPaaS provides SMS and verification capabilities that developers can integrate into existing applications and workflows.
- 8×8 CPaaS SMS API enables developers to send SMS programmatically through your applications. This can support use cases such as OTP delivery, notifications, alerts and other transactional messages.
Instead of managing SMS manually, your application can trigger messages through API requests as part of an automated workflow.
- 8×8 Verification API follows the managed Verify API approach, allowing businesses to verify users through codes delivered via SMS, voice or email. By handling more of the verification workflow for you, it can simplify implementation while giving developers additional channels to use when SMS alone is not suitable.
- The 8×8 Developer Portal provides API references and integration guides to help developers connect 8×8 communication services with your applications and workflows.
This gives technical teams a starting point for understanding available endpoints, authentication and integration requirements before building an implementation.
Need help with your SMS API integration process? Talk to our experts to explore the right SMS and verification solution for your application and use case.
Frequently Asked Questions
- How do I send an SMS through an API?
You typically send an authenticated HTTP request to an SMS API endpoint containing the recipient’s phone number and message content. The API provider then processes the request and routes the SMS to the recipient.
- How do I integrate an OTP verification API?
An OTP integration generally involves generating a secure code, storing its verification state, sending it through an SMS or verification API, validating the user’s submitted code and enforcing expiry and attempt limits.
- What is an SMS API?
An SMS API is a software interface that allows applications to send and manage SMS messages programmatically through HTTP requests.
- What is an OTP API?An OTP API provides functionality for sending and managing one-time passwords as part of user verification and authentication workflows. Depending on the provider, OTPs may be delivered through SMS, voice or other channels.
- What are delivery receipts?Delivery receipts are status notifications that indicate what happened to a submitted SMS, such as whether it was delivered, failed or is still being processed.
- How long should an OTP remain valid?OTP validity should be short enough to reduce security risk while giving the user sufficient time to receive and enter the code. The appropriate duration depends on the application’s risk profile and authentication requirements.
- Is SMS OTP secure enough for every application?SMS OTP is a familiar and widely supported verification method, but it is not appropriate for every authentication scenario. Higher-risk applications may require stronger authentication methods or additional security controls.
