### Answer For SMS with a Twilio number connected via SIP trunk, you have a couple of options: For outbound SMS from your backend: You can use the [Create Outbound SMS API]() to programmatically send SMS. [(1) ]() For the built-in SMS function during calls: You need to set up the SMS function by providing your Twilio Account SID and Auth Token in the Retell dashboard under "Setup SMS Function" in Advanced Add-Ons. Important limitation noted in community discussions: There was initially some confusion about SMS support for custom telephony numbers. One user reported getting an error when using the

send_sms

function with a SIP-trunked number, but later confirmed it started working. [(2) ]() For inbound SMS: When you set up your SIP trunk in Twilio, voice traffic goes to the trunk, but the endpoints for Programmable Messaging are still available for configuration separately. You can configure Twilio's messaging webhook to point to your own backend (e.g., a Make webhook) to receive inbound SMS. [(3) ]() So to directly answer your question: SIP trunking handles voice traffic, while SMS configuration remains separate in Twilio. You can use Retell's API from your backend for outbound SMS, and configure Twilio's messaging webhooks independently for inbound SMS routing.

​ To learn more, see the sources I considered: [[1] Send & receive SMS](https://docs.retellai.com/deploy/enable-sms) [[2] Error in sending SMS](https://discord.com/channels/1186424376300679209/1412053422944817313) [[3] Discord message from @mushrooooom on 2025-06-26](https://discord.com/channels/1186424376300679209/1186462578310910112/1387632326682611873) ​

Feel free to tag @AI Evy with additional questions. I also appreciate feedback, it helps me improve.

### Answer No, the regular Create Chat endpoint does not support

from_number

or

to_number

parameters. [(1) ]() If you need to send SMS with phone numbers, you should use the Create Outbound SMS endpoint instead, which requires both

from_number

and

to_number

as required parameters. [(2) ]()

{
  "from_number": "+12137771234",
  "to_number": "+14155551234"
}

The regular Create Chat API is for API-based chat sessions without telephony, while Create Outbound SMS is specifically designed for SMS conversations with phone number routing.

​ To learn more, see the sources I considered: [[1] Create Chat](https://docs.retellai.com/api-references/create-chat) [[2] Create Outbound SMS](https://docs.retellai.com/api-references/create-sms-chat) ​

Feel free to tag @AI Evy with additional questions. I also appreciate feedback, it helps me improve.