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Telnyx enforces strict caller ID validation to prevent spoofing and ensure compliance with telecommunications regulations. Invalid caller ID numbers are rejected with a 403 Caller Origination Number is Invalid D35 SIP response.

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Number format requirements

The accepted caller ID format depends on the connection’s localization setting and call destination.

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USA localization

Accepted formats: 
3129457420          # National format
13129457420         # 11-digit format
+13129457420        # E.164 format

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International localization

For non-USA localization (e.g., Ireland): 
840-1234            # National format
01-840-1234         # Country-specific format
+353-1-840-1234     # E.164 format

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Cross-border calls

When calling outside the localization country, only E.164 format is accepted: 
+1 (country code) + area code + number
Example: A connection with Ireland localization calling the USA must use +13129457420.

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Caller ID override

Connections with Caller ID Override enabled can send any format for outbound calls, bypassing standard validation rules.

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SIP header priority

Telnyx checks the following SIP headers in order to determine the caller ID:
  1. P-Preferred-Identity (User part)
  2. P-Asserted-Identity (User part)
  3. Remote-Party-Id (User part)
  4. FROM (User part)
When multiple headers are present, only the highest priority header is used.

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Caller ID anonymization

To anonymize the caller ID, include the Privacy: id header in the SIP request: 
Privacy: id
Requirements:
  • A valid origination number must still be provided
  • Invalid numbers with this header trigger the D35 error
  • Toll-free and emergency number calls cannot be anonymized
Recipients always see the actual caller ID for:
  • Emergency services (911, etc.)
  • Toll-free numbers

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International spoofing restrictions

Outbound calls to international destinations with spoofed caller IDs are rejected with a 503 error response. This allows implementing fallback routing logic on the client side.

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Validation behavior

If no localization is configured and a number appears invalid, Telnyx defaults to USA validation rules. Failed validation returns a 404 Invalid Destination response.

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Configuration

Set the connection localization in the Mission Control Portal under Connection Outbound Settings, or via the API: 
{
  "outbound": {
    "localization": "US"
  }
}

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Troubleshooting

ErrorCauseSolution
403 D35Invalid caller ID formatUse E.164 format or match localization requirements
404Invalid destinationVerify the called number is in E.164 format
503International spoofingUse a valid origination number for the destination country