Caller ID Number (CID) is a service that displays your phone number to the phone of the individual you are calling on an outbound call. Telnyx upholds a strict policy around the handling of Caller ID Numbers for outbound calls. 

All outbound calls with Caller ID Numbers that are not valid will be rejected with a SIP response code of “403 Caller Origination Number is Invalid D35”.

Supported Number Formats:

When creating a SIP Connection, by default, localisation is set to USA. This means our system will accept calls in national, 11 digit or +E.164 format.


If my localisation is set to United States and I primarily want to reach destinations in Ireland and the USA, I would be allowed dial the following formats.

United States:
Accepted dialing formats - national, 11 digit and +E.164.

  • 3129457420

  • 1312945720

  • +1312945720


Accepted dialling formats - +E.164 only.

  • +353-1-840-1234

If the localisation was set to Ireland and I wanted to dial Irish and American numbers:

Accepted dialing formats - national, 11 digit and +E.164.

  • 840-1234

  • 01-840-1234

  • +353-1-840-1234

United States:

Accepted dialling formats - +E.164 only.

  • +13129457420

If the Connection’s Caller ID Override is set and used for an outbound call

  1. Send any format if you setup override in your Connection's Outbound settings.

How Telnyx handles the Caller ID Number:

  • The Localisation Country can be set through the Telnyx Mission Control Portal under the Connection's Outbound Settings.

  • If a Connection does not have a Localisation Country and the number dialled appears to be invalid, Telnyx will attempt to validate the number using USA as the Localisation Country.

  • If the previous check failed, Telnyx will return a 404 invalid destination response.

Which headers from the SIP INVITE can carry the Caller ID Number?

Below listed is the following SIP headers that are accepted for Caller ID, ordered by priority (1 highest and 4 lowest priority)

1. P-Preferred-Identity User
2. P-Asserted-Identity User
3. Remote-Party-Id User
4. FROM User

In the case that more than one header is provided the highest priority header is followed.

How to Anonymize Caller ID Number 

The following SIP header needs to be provided along with a caller ID that obeys the specifications described in the two points above.

Privacy: id

As well as the above, you need to ensure that the user@domain parts of the URI's in any of the above SIP headers contain "anonymous". If any caller ID header exists with the user or domain part of the SIP URI without "anonymous", calls will be rejected with 403 Caller Origination Number is Invalid D35

Good Examples:

The display name including user/domain of the SIP URI are all anonymized

From: "Anonymous" <sip:anonymous@anonymous.invalid>;tag=12345-6789
P-Asserted-Identity: "Anonymous" <sip:anonymous@anonymous.invalid>
P-Preferred-Identity: "Anonymous" <sip:anonymous@anonymous.invalid>

Bad Examples:

Where the users name or IP/domain is exposed and not anonymized

From: "Anonymous" <sip:john@>;tag=12345-6789
P-Asserted-Identity: "Anonymous" <sip:anonymous@>
P-Preferred-Identity: "Anonymous" <sip:john@anonymous.invalid>

Where the display name is exposed and not anonymized

From: "John" <sip:anonymous@anonymous.invalid>;tag=12345-6789
P-Asserted-Identity: "John" <sip:anonymous@anonymous.invalid>
P-Preferred-Identity: "John" <sip:anonymous@anonymous.invalid>

Note: The Caller ID of an outbound call will be anonymized downstream as the "Privacy: id" SIP Header is present. 

Note: If the number you wish to call anonymously is a toll-free number, your Caller ID number will not be anonymized. The owner of the toll free number will be paying for the call and therefore has the right to know who is calling them. This also applies when dialling emergency numbers.

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