Azure communication Service-OTP based

Hello @Rakesh

Yes, you can use Azure Communication Services (ACS) SMS to send an OTP to a user's mobile number from your custom application.

The typical flow would be:

User requests login/change → your backend generates an OTP → ACS sends the OTP by SMS → user enters the OTP → your backend validates it → the requested action is allowed

One important point: ACS provides SMS delivery; it doesn't manage the OTP lifecycle for your application. If you implement this yourself, your backend needs to securely generate the OTP, associate it with the user/session, set a short expiration time, limit retries/resends, prevent reuse, and validate it before authorizing the operation.

You would normally acquire/configure an SMS-capable sender number in your ACS resource and then use the ACS SMS SDK or REST API from your backend to send the message. Keep the ACS credentials on the server side rather than exposing them in the client application. Microsoft documents Access Key and Azure RBAC authentication for ACS SMS operations.

Regulatory requirements also depend on the destination country and sender type. For example, if you're sending application-to-person SMS to U.S. users through a 10-digit long code (10DLC), Microsoft requires brand and campaign registration. An existing voice number that needs SMS enabled must first go through the applicable 10DLC registration and association process.

If this OTP is specifically for user authentication/MFA, consider Microsoft Entra External ID rather than building the entire verification system yourself. External ID supports SMS one-time passcodes as an MFA method for customer applications, including native authentication scenarios.

So the choice is essentially:

• ACS SMS - appropriate when you want complete control over a custom OTP workflow and your application handles OTP generation/validation.
• Microsoft Entra External ID - preferable when the requirement is authentication/MFA, and you want Microsoft identity services to handle the authentication workflow.

If you can confirm which country the recipient mobile numbers are in and whether this OTP is for authentication/MFA or just transaction verification, we can recommend the appropriate ACS number type/registration and implementation.

References:

SMS overview - Azure Communication Services

10DLC brand and campaign registration - Azure Communication Services

Azure Communication Services authentication

Authentication options - Microsoft Entra External ID

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