Sinch Verification API

Overview

The Sinch Verification API verifies phone numbers through SMS OTP, Flashcall (missed call CLI), Phone Call (spoken OTP), Data (carrier-level), and WhatsApp OTP. Used for registration, 2FA, and number ownership confirmation.

Base URL: https://verification.api.sinch.com
URL path prefix: /verification/v1/
Auth: Application Key + Secret (NOT project-level OAuth2 — see Authentication Guide)

Agent Instructions

Before generating code, you MUST ask the user:

1. Which verification method? — sms, flashcall, callout, seamless, or whatsapp
2. SDK or direct HTTP? — If SDK, which language?

Do not assume defaults or skip these questions. Wait for answers before generating code.

For SDK syntax and setup, see sinch-sdks. For direct HTTP, use the API Reference (Markdown) for request/response schemas.

Getting Started

Agent Credentials handling

Store credentials in environment variables — never hardcode application keys or secrets in commands or source code:

export SINCH_APPLICATION_KEY="your-application-key"
export SINCH_APPLICATION_SECRET="your-application-secret"

Authentication

Ensure that authentication headers are properly set when making API calls. The Verification API uses Application Key + Application Secret (from your Sinch dashboard app), not project-level OAuth2:

-u "$SINCH_APPLICATION_KEY:$SINCH_APPLICATION_SECRET"

See sinch-authentication skill for dashboard setup.

Three auth methods are supported:

Minimum auth level is configurable in the Sinch Dashboard — requests below that level are rejected. See the Authentication Guide for signing details.

SDK Setup

See sinch-sdks for installation and client initialization across all languages. All SDKs initialize with applicationKey + applicationSecret (not project credentials).

Canonical Example — Start SMS Verification

# Uses Basic Auth (-u) for simplicity. Use Application Signed Requests in production.
curl -X POST \
  "https://verification.api.sinch.com/verification/v1/verifications" \
  -u "$SINCH_APPLICATION_KEY:$SINCH_APPLICATION_SECRET" \
  -H 'Content-Type: application/json' \
  -d '{
    "identity": { "type": "number", "endpoint": "+12025550134" },
    "method": "sms"
  }'

Response includes id (verification ID), sms.template, sms.interceptionTimeout, and _links with localized URLs for status/report actions.

Key Concepts

Verification Methods

Core Model

• Identity: Always { "type": "number", "endpoint": "+E164_NUMBER" }
• Verification ID: Returned on start. Used to report code or query status.
• Reference: Optional unique tracking string in start request. Queryable via status endpoint.
• Statuses: PENDING | SUCCESSFUL | FAIL | DENIED | ABORTED | ERROR
• Failure reasons (most common): Invalid code, Expired, Fraud, Blocked, Denied by callback. Full list in the API Reference.

API Endpoints

All endpoints documented in the Verification API Reference.

Start Verification

POST /verification/v1/verifications

Set method to sms, flashcall, callout, seamless, or whatsapp. Optional fields:

• reference — unique tracking string, passed to all events
• custom — arbitrary text (max 4096 chars), passed to all events
• Accept-Language header — controls SMS language (default en-US)

Method-specific options (backend-originated signed requests only): smsOptions, flashCallOptions, calloutOptions, whatsappOptions. See the API Reference for full schemas.

Report Verification

Report by identity: PUT /verification/v1/verifications/number/{endpoint}
Report by ID: PUT /verification/v1/verifications/id/{id}

Body includes method and a method-specific object with the user's input:

• SMS / Phone Call / WhatsApp: { "method": "sms", "sms": { "code": "1234" } } (replace method name + key accordingly)
• FlashCall: { "method": "flashcall", "flashCall": { "cli": "+46000000000" } } — the cli is the full international caller ID from the incoming missed call

Get Verification Status

By ID: GET /verification/v1/verifications/id/{id}
By method + number: GET /verification/v1/verifications/{method}/number/{endpoint}
By reference: GET /verification/v1/verifications/reference/{reference}

Note: The by-identity endpoint requires {method} in the path — it is NOT /verifications/number/{endpoint}.

Common Patterns

Standard Verification Flow

1. Start — POST /verification/v1/verifications with identity + method → receive verification id
2. Report — User receives code/call → PUT /verification/v1/verifications/id/{id} with the code/CLI
3. Check status — GET /verification/v1/verifications/id/{id} → confirm SUCCESSFUL

If the code expires or verification fails, you cannot re-report — start a new verification.

Webhooks (Callbacks)

For production flows, configure a callback URL in the Sinch Dashboard. The API sends:

• VerificationRequestEvent — fired when a verification starts. Respond with action: allow or action: deny to approve/reject.
• VerificationResultEvent — fired when a verification completes (success or failure). Use for logging, analytics, or triggering downstream actions.

Callbacks are signed — verify signatures using Callback Signing.

Gotchas and Best Practices

1. Auth is Application Key + Secret, not OAuth2. Do not use project-level credentials.
2. Use Application Signed Requests in production. Application auth protects integrity of a request
3. Base64-decode the secret before signing. The dashboard value is base64-encoded.
4. FlashCall auto-intercepts on Android only. iOS/JS users must manually enter the incoming number. Android SDK is required to intercept calls.
5. Method availability varies by country. SMS is the most widely available.
6. Codes expire. Configurable via smsOptions.expiry. Start a new verification if expired — you cannot re-report on a completed/expired verification.
7. Report by ID is more precise than reporting by phone number.
8. Rate limit: avoid rapid re-verification of the same number. Implement backoff.
9. Data verification requires account manager and mobile data (not Wi-Fi).
10. SMS language may be overridden by carrier compliance (e.g., US shortcode requirements).

Links

• Verification API Reference (Markdown)
• Verification OpenAPI Spec (YAML)
• Authentication Guide
• Application Signed Requests
• Callback Signing
• Getting Started Guide
• Node.js SDK Reference
• Python SDK Reference
• Java SDK Reference
• .NET SDK Reference
• LLMs.txt (full docs index)