Caller ID Policy

Configure caller ID policy on Telnyx SIP Trunking connections. Allow, override, or block From and P-Asserted-Identity values on outbound calls.

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.

Number format requirements

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

USA localization

Accepted formats:

3129457420          # National format
13129457420         # 11-digit format
[phone]        # E.164 format

International localization

For non-USA localization (e.g., Ireland):

840-1234            # National format
01-840-1234         # Country-specific format
[phone]     # E.164 format

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 [phone].

Caller ID override

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

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.

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

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.

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.

Configuration

Set the connection localization in the Mission Control Portal under Connection Outbound Settings, or via the API:

• PATCH /v2/credential_connections/
• PATCH /v2/fqdn_connections/
• PATCH /v2/ip_connections/

{
  "outbound": {
    "localization": "US"
  }
}

Troubleshooting

Error	Cause	Solution
403 D35	Invalid caller ID format	Use E.164 format or match localization requirements
404	Invalid destination	Verify the called number is in E.164 format
503	International spoofing	Use a valid origination number for the destination country