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Communication is not bound to voice only. SMS is a common way to communicate with customers. This guide will show you how to enable SMS with Retell Twilio numbers or bring your own Twilio number.
SMS capabilities are available for Retell Twilio numbers and custom telephony numbers that have passed A2P applications. Telnyx is not supported yet. For other custom telephony providers, please check your own telephony provider’s documentation on how to enable SMS.
Current SMS capabilities are limited to standard US numbers only. Toll-free numbers are not supported.

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Enable SMS capabilities

This step is a prerequisite for sending SMS from your own number during calls and setting up two-way SMS conversations. Receiving SMS during calls works out of the box for Retell Twilio numbers without this step.
If you only need to send SMS during calls and want to skip the A2P application, you can choose to send from an SMS-approved Retell number instead. See Send SMS during call for details.

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Option 1: Enable SMS for Retell Twilio Numbers

To enable SMS, it will go through the following application and approval steps:
  • Get approved for a business profile (free)
  • Get approved for the brand based on business profile ($4 one time application fee for low-volume, $45 one time application fee for standard)
  • Get approved for an SMS campaign ($15 one-time application fee)
It can take around 2-3 weeks for the entire application, sometimes longer, as it’s a manual review process on the telephony provider side. We will notify you via email when the application is approved or rejected with the reason. If your application is stuck in pending review for over a month, please reach out to support and we will help escalate.

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Detailed steps

1

Get approved for a business profile

You can reuse an existing business profile or create a new one here. Follow the instructions at Business Profile to get approved for a business profile.
2

Select a brand type and get approved for the brand

Only one brand can be created per business profile and it cannot be changed once created. If you need to change the brand type, you have to create a new business profile.
Here you get to select two types of brands:
  • Low-volume:
    • $4 one time application fee
    • send fewer than 6,000 message segments per day to the US (2,000 message segments per day to T-Mobile)
  • Standard:
    • $45 one time application fee
    • SMS limit may fall between 6,000 and 400,000 message segments per day to the US (2,000 - 200,000 per day to T-Mobile)
Depending on your company’s type, you might be asked to fill out additional information.
3

Get approved for an SMS campaign

Here you will fill out the use case, and sample messages you intend to send. Please note that there are very strict rules on SMS, and if your actual traffic doesn’t match the sample messages, your number / telephony subaccount might be suspended. Therefore, please be honest and provide answers that match your intended use case. If you ever need to send different SMS messages, you can create a new campaign (check FAQ for more details).

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Rejected application FAQ

There are a few common reasons for application rejection:
  • the campaign application did not provide detailed information on the use case and sample messages.
  • Opt in workflow was not explained clearly.
  • Business profile or brand was not approved, due to missing or incorrect business information.
Feel free to read more at Twilio’s A2P 10DLC documentation.
Please check the rejection reason and update your application accordingly. If you are rejected at a later stage, you might be able to reuse the previous steps. For example, if your business profile and brand are approved, but the SMS campaign is rejected, you can reuse the business profile and brand approval, and create a new SMS campaign application.
No, you will not get a refund, as the charge is for the telephony provider’s manual review process, and that will be charged regardless of the application outcome.
You can delete the SMS capability on the number (this will not delete your approved business profile and brand, and campaign, those can still be reused), and create a new SMS application with a new campaign while reusing the business profile and brand.

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Option 2: Bring Your Own Twilio Number

If you already have a Twilio number with SMS capabilities enabled, you can integrate it with Retell by following these steps:
1

Click on Setup SMS Function in Advanced Add-Ons

Navigate to your phone number settings in the Retell dashboard and click on the “Setup SMS Function” button under Advanced Add-Ons.
2

Provide Your Twilio Credentials

Enter the following information from your Twilio account:
  • Account SID: Your unique Twilio Account identifier
  • Twilio Auth Token: Your Twilio authentication token for API access
You can find these credentials in your Twilio Console under Account Info.
3

Configure SMS Settings

Once your credentials are verified, your Twilio number will be integrated with Retell’s SMS capabilities. You can then use it for:
  • Sending SMS during calls
  • Setting up two-way SMS conversations (requires enabling useInboundWebhookOnNumber on Twilio dashboard for your custom telephony numbers)
  • Make sure your Twilio number already has SMS capabilities enabled and is compliant with SMS regulations before integration
  • To enable two-way SMS conversations, enable the useInboundWebhookOnNumber option in your phone number configuration on Twilio dashboard
  • For more information about A2P 10DLC compliance, see Twilio’s A2P 10DLC documentation

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Option 3: Use an SMS-Approved Retell Number (No A2P Required)

If you only need to send SMS during active phone calls and want to skip the A2P application entirely, you can send from Retell’s pool of SMS-approved numbers. No setup or approval steps are needed — select this option when configuring your SMS node or tool, and you can start sending right away. A number from the pool is automatically selected for each call, and all SMS sent within the same call use the same number.
When sending from an SMS-approved Retell number, the message content is a preset template provided by Retell — you cannot customize the text or use a prompt. This option is for in-call SMS only and does not support two-way SMS conversations.

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Send SMS during call

You can configure agents to send SMS messages to the user during an active phone call. Refer to the following docs for setup details:

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Receive SMS during call

Agents can receive and understand SMS messages during an active phone call — even if the agent has not sent any SMS. This allows users to send supplementary information mid-conversation, such as a photo of a document, a screenshot, or a reference number. For Retell Twilio numbers, this works out of the box without enabling SMS. For custom telephony numbers that have passed A2P applications, this is also supported. Multimedia (MMS) is supported in addition to plain text: users can send images, audio, and video as carrier-supported MMS attachments. Whatever is delivered to Retell as part of the message is incorporated into the ongoing voice conversation so the agent can respond with that context.

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Set up two-way SMS conversation

Once your SMS capability is enabled for the number, you can set up a two-way SMS conversation by attaching chat agents to the number, where the number will be able to receive SMS and reply back to user. Once the chat agent is attached, the inbound will already work. Inbound webhook can be used to filter and add context to inbound SMS. Read more at Inbound Webhook. For making outbound SMS, you can do that in the dashboard by clicking on the Make an outbound SMS button, or use the Create Outbound SMS API to programmatically send it.