Caller ID number changing unexpectedly in Retell + Twilio setup

Support Help
1

I’m running into a strange issue with the phone number that appears as the caller ID, and I’d like to understand if this is expected behavior or a configuration problem.

I configured my Retell phone number using Twilio and tested it several times before putting it into production. During those initial tests, everything worked correctly — the exact number I registered in Retell showed up on my phone when I received calls.

A few days later, I went back to update and optimize my AI agent script. After that, I tested the calls again. To my surprise, the number showing on my phone was no longer the one I registered. Instead, I received calls from completely different numbers that I don’t recognize.

I’m based in Brazil and tested using my Brazilian phone number. My Retell number is configured with a +1 country code, but the incoming calls showed Brazilian numbers with country code +55 (area code 19). I tested two more times, and each time the caller ID changed again. Some of these numbers were even flagged as spam.

Is this normal behavior when using Retell with Twilio, or could there be a configuration issue on my side that’s causing the caller ID to rotate?

Any guidance would be appreciated.

2

The behavior you’re experiencing—where the caller ID changes to different Brazilian numbers instead of your registered +1 number—is not standard Retell AI behavior and indicates a configuration issue.

Phone Number Masking Configuration

Retell AI supports phone number masking through Twilio’s verified caller ID feature(1). If you want your personal phone number or trusted business number to display to the callee, you need to configure verified phone numbers in Twilio to serve as the caller ID(1).

To set this up properly:

  1. Go to the Verified Caller IDs page in Twilio(1)
  2. Click “Add a new Caller ID”(1)
  3. Enter the desired phone number to verify and select the verification method(1)
  4. The number will receive an OTP Authentication code for verification(1)
  5. Once verified, configure SIP Header Rules to display the caller ID(1)

SIP Header Manipulation for Caller ID

In the Twilio Console, navigate to Elastic SIP Trunking → Trunks → [your trunk] → Termination(1). Under Header Manipulation, create a policy with the following settings(1):

  • SIP header field: From number(1)
  • Action: Replace with(1)
  • Value: Your caller ID in E.164 format (e.g. +18881230987)(1)

After creating the policy, select it from the dropdown in the Termination tab(1). Any number imported from this Twilio trunk into your Retell account will then use your verified caller ID(1).

Regional Considerations for Brazil

Based on community reports, there may be carrier-specific restrictions when calling Brazilian numbers from international caller IDs(2). Some Brazilian carriers block or restrict calls to certain number ranges when originating from outside specific metropolitan areas(2). The rotating Brazilian numbers (+55) you’re seeing suggest that Twilio may be attempting to route calls through local gateways, which could explain the changing caller IDs.

Recommended Actions

  1. Verify your SIP Header Manipulation policy is correctly configured in Twilio(1)
  2. Ensure you have at least one number purchased from Twilio in your SIP trunk for the caller ID to be applied(1)
  3. Check your Twilio logs for any error codes that might indicate routing issues(3)
  4. Contact Twilio support to understand any restrictions on your specific phone number configuration(2)

The fact that it worked initially but changed after updating your agent script suggests the issue may be related to your Twilio trunk configuration rather than the Retell agent itself.