Trusted Calling with SHAKEN/STIR

Robocalls are calls that are created by an auto-dialer, and typically play pre-recorded messages. Fraudsters use robocalls in combination with spoofing (falsifying caller IDs) to acquire something of value from their victims. In 2020, there were 48.9 billion robocalls in the United States, leading to an erosion of trust in the telephone network and a decline in call answer rates from unidentified phone numbers.

SHAKEN stands for Signature-based Handling of Asserted Information using toKENs. It is a specification designed by the Alliance for Telecommunications Industry Solutions (ATIS) to fight caller ID spoofing. STIR (Secure Telephone Identity Revisited) is a protocol developed by the Internet Engineering Task Force (IETF) to enable end-to-end call authentication, but the protocol is very broad and doesn't ensure that different providers will be able to verify each others' calls. SHAKEN is a set of implementation details that follows the STIR protocol, but streamlines specifications to increase the likelihood of carrier interoperability. This is why you will often see the technology referred to as "SHAKEN/STIR" or "STIR/SHAKEN".

Essentially, SHAKEN/STIR uses encrypted digital signatures to share information about the caller to each provider along a call's path from caller to recipient, such as the caller's identity and whether the caller has the right to use the phone number they provided as the caller ID.

To learn more about the implementation of SHAKEN/STIR, read the following resources:

• Everything You Need to Know About SHAKEN/STIR Right Now (Twilio Blog)
• Combating Spoofed Robocalls with Caller ID Authentication (FCC)
• TRACED Act Implementation (FCC)

Twilio, like all major carriers in the United States, has signing and verifying privileges. Keep reading to learn about the product changes you can expect with SHAKEN/STIR.

• For Programmable Voice customers, a new parameter will be present in incoming webhooks and outgoing calls: StirVerstat.
• For Elastic SIP Trunking customers, there is a new header called X-Twilio-VerStat, and a new Identity header with the SHAKEN PASSporT.

To understand the possible values for the StirVerstat parameter/X-Twilio-VerStat header, you will first need to understand the three different attestation levels:

• A : the highest attestation given by the originating service provider to indicate that the caller is known and has the right to use the phone number as the caller ID
• B : the customer is known, it is unknown if they have the right to use the caller ID being used
• C : it doesn't meet the requirements of A or B including international calls.

The table below describes the possible values for the StirVerstat parameter/X-Twilio-VerStat header.

Twilio Programmable Voice and Elastic SIP Trunking now perform SHAKEN/STIR verification on incoming calls to your Twilio local phone numbers. It can be used to display a trust indicator or to make a routing decision, such as bypassing a voice captcha or IVR and directing the call to an end user.

A verified call that has been given the highest attestation under SHAKEN/STIR means that the carrier that originated the call both (1) knows the identity of the caller, and (2) knows the caller has the right to use the phone number as the caller ID.

Note: The StirVerstat parameter is only present in the incoming call webook when the incoming call has SHAKEN PASSporT identity headers. The X-Twilio-VerStat header is only present in SIP INVITEs for incoming calls that have SHAKEN PASSporT identity headers.

When your application receives a request webhook that has a StirVerstat parameter, Twilio will implicitly pass the StirVerstat to the Client when you <Dial><Client>. The information in the StirVerstat parameter can be used to display a trust indicator to the recipient when an incoming call from the public telephone network has been verified under the SHAKEN/STIR framework.

• The JavaScript Client now has: Connection.CallerInfo.isVerified
• The Android and iOS Mobile SDKs now have the CallerInfo object and TVOCallerInfo class to represent information about the caller.

The Status Callback StirStatus optional parameter will inform you of the attestation Twilio gave your call to the public telephone network. If the call is forwarded (functionality coming soon), this will be attestation of the incoming call that was forwarded.

• <Dial>
• Call Resource
• Voice Webhooks

You may encounter situations in which an incoming call to your Twilio phone number needs to be forwarded to another number. If you want to maintain the original caller's CallerID for the forwarded leg of the call and facilitate SHAKEN/STIR verification, you will need to pass a CallToken from the parent leg of the call to the forwarded leg.

When one of your Twilio phone numbers receives an incoming call, Twilio sends a webhook to your application (please note this will only happen if you use POST). The request body of this webhook contains a CallToken property. The CallToken contains any SHAKEN/STIR and DIV (diversion) PASSporTs contained in the SIP headers of the incoming call.

In order to forward a call received by your Twilio number, you must:

1. Use the value of the CallToken from the inbound call's incoming webhook as the CallToken parameter when creating a new Call Resource or creating a new Conference Participant.
2. Use the original caller's phone number as the From parameter when creating the new Call Resource or Conference Participant.

Example call forwarding scenario

A caller (+12222222222) dials your Twilio phone number (+15555555555) and you need to forward the call to a new phone number (+18888888888).

Upon receiving the incoming call, your application will receive a webhook with the following request body:

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{
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  Called: '+15555555555',
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  ToState: 'AL',
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  CallerCountry: 'US',
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  Direction: 'inbound',
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  CallerState: 'PA',
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  ToZip: '33333',
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  CallSid: 'CAXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX',
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  To: '+15555555555',
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  CallerZip: '88888',
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  ToCountry: 'US',
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  StirVerstat: 'TN-Validation-Passed-A',
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  CalledZip: '33333',
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  ApiVersion: '2010-04-01',
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  CalledCity: 'DOTHAN',
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  CallStatus: 'ringing',
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  From: '+12222222222',
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  AccountSid: 'ACXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX',
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  CalledCountry: 'US',
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  CallerCity: 'PHILADELPHIA',
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  StirPassportToken: 'STIR_TOKEN_IN_JWT_FORM',
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  ToCity: 'DOTHAN',
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  FromCountry: 'US',
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  Caller: '+12222222222',
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  FromCity: 'PHILADELPHIA',
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  CalledState: 'AL',
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  FromZip: '88888',
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  FromState: 'PA'
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  CallToken: '%7B%20%22parentCallInfoToken%22%3A%22eyJhbGciOiJFUzI1NiJ9.eyJjYWxsU2lkIjoiQ0FYWFhYLi4uLiIsImZyb20iOiIrMTIyMjIyMjIyMjIiLCJ0byI6IisxNTU1NTU1NTU1NSIsImlhdCI6IjE2MzU5NTc0MjMifQ.jVOxmCbSxxKg2fuzvDT_-PTStRRw_TrWgdh2QaZNzHQwvgwO6Qk_FRPFPYguJQn19x0yZqltPQfHwql4FJt_7g%22%2C%0A%20%20%22identityHeaderTokens%22%3A%5B%0A%20%20%20%20%20%20%22eyJhbGciOiJFUzI1NiIsInBwdCI6InNoYWtlbiIsInR5cCI6InBhc3Nwb3J0IiwieDV1IjoiaHR0cHM6Ly9leGFtcGxlLmNvbS9zb21lQ2VydGlmaWNhdGUucGVtIn0.eyJhdHRlc3QiOiJBIiwiZGVzdCI6eyJ0biI6WyIxNTU1NTU1NTU1NSJdfSwiaWF0IjoxNjM1OTU3NDIzLCJvcmlnIjp7InRuIjoiMTIyMjIyMjIyMjIifSwib3JpZ2lkIjoic29tZS1HVUlELWhlcmUifQ.ygPO2sImJR9MSqoRVD0CvnB2euv3RUYdupNEFS3wpgecs-yi8SU8FtYkkDypn7DC-siwdPY6vvVIx39y5Nb1sg%3Binfo%3D%3Chttps%3A%2F%2Fexample.com%2FsomeCertificate.pem%3E%3Balg%3DES256%3Bppt%3Dshaken%22%0A%20%20%20%20%20%20%5D%0A%7D'
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}

To forward the call, you will retrieve the CallToken value from the incoming webhook (above) and use this value as the CallToken parameter when you create a new Call Resource.

The code sample below shows how to do that with your chosen Helper Library, cURL, and the Twilio CLI. Please be aware that this code sample only shows how to use the API to create the outbound call leg through which attestation is passed. In order to connect the inbound and outbound calls so that the two parties can speak with each other, you will need to implement a call flow for both calls using Programmable Voice Queues or Conferences. Using the <Dial> instruction is not possible for this call flow.

Note: You use the original caller's number (+12222222222) as the From parameter.

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// Download the helper library from https://www.twilio.com/docs/node/install
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const twilio = require("twilio"); // Or, for ESM: import twilio from "twilio";
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// Find your Account SID and Auth Token at twilio.com/console
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// and set the environment variables. See http://twil.io/secure
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const accountSid = process.env.TWILIO_ACCOUNT_SID;
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const authToken = process.env.TWILIO_AUTH_TOKEN;
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const client = twilio(accountSid, authToken);
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async function createCall() {
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  const call = await client.calls.create({
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    callToken:
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      "%7B%20%22parentCallInfoToken%22%3A%22eyJhbGciOiJFUzI1NiJ9.eyJjYWxsU2lkIjoiQ0FYWFhYLi4uLiIsImZyb20iOiIrMTIyMjIyMjIyMjIiLCJ0byI6IisxNTU1NTU1NTU1NSIsImlhdCI6IjE2MzU5NTc0MjMifQ.jVOxmCbSxxKg2fuzvDT_-PTStRRw_TrWgdh2QaZNzHQwvgwO6Qk_FRPFPYguJQn19x0yZqltPQfHwql4FJt_7g%22%2C%0A%20%20%22identityHeaderTokens%22%3A%5B%0A%20%20%20%20%20%20%22eyJhbGciOiJFUzI1NiIsInBwdCI6InNoYWtlbiIsInR5cCI6InBhc3Nwb3J0IiwieDV1IjoiaHR0cHM6Ly9leGFtcGxlLmNvbS9zb21lQ2VydGlmaWNhdGUucGVtIn0.eyJhdHRlc3QiOiJBIiwiZGVzdCI6eyJ0biI6WyIxNTU1NTU1NTU1NSJdfSwiaWF0IjoxNjM1OTU3NDIzLCJvcmlnIjp7InRuIjoiMTIyMjIyMjIyMjIifSwib3JpZ2lkIjoic29tZS1HVUlELWhlcmUifQ.ygPO2sImJR9MSqoRVD0CvnB2euv3RUYdupNEFS3wpgecs-yi8SU8FtYkkDypn7DC-siwdPY6vvVIx39y5Nb1sg%3Binfo%3D%3Chttps%3A%2F%2Fexample.com%2FsomeCertificate.pem%3E%3Balg%3DES256%3Bppt%3Dshaken%22%0A%20%20%20%20%20%20%5D%0A%7D",
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    from: "+12222222222",
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    to: "+18888888888",
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    url: "https://www.example.com",
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  });
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  console.log(call.sid);
20
}
21

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createCall();

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{
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  "account_sid": "ACaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa",
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  "annotation": null,
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  "answered_by": null,
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  "api_version": "2010-04-01",
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  "caller_name": null,
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  "date_created": "Tue, 31 Aug 2010 20:36:28 +0000",
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  "date_updated": "Tue, 31 Aug 2010 20:36:44 +0000",
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  "direction": "inbound",
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  "duration": "15",
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  "end_time": "Tue, 31 Aug 2010 20:36:44 +0000",
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  "forwarded_from": "+141586753093",
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  "from": "+12222222222",
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  "from_formatted": "(415) 867-5308",
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  "group_sid": null,
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  "parent_call_sid": null,
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  "phone_number_sid": "PNaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa",
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  "price": "-0.03000",
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  "price_unit": "USD",
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  "sid": "CAaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa",
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  "start_time": "Tue, 31 Aug 2010 20:36:29 +0000",
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  "status": "completed",
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  "subresource_uris": {
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    "notifications": "/2010-04-01/Accounts/ACaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa/Calls/CAaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa/Notifications.json",
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    "recordings": "/2010-04-01/Accounts/ACaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa/Calls/CAaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa/Recordings.json",
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    "payments": "/2010-04-01/Accounts/ACaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa/Calls/CAaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa/Payments.json",
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    "events": "/2010-04-01/Accounts/ACaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa/Calls/CAaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa/Events.json",
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    "siprec": "/2010-04-01/Accounts/ACaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa/Calls/CAaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa/Siprec.json",
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    "streams": "/2010-04-01/Accounts/ACaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa/Calls/CAaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa/Streams.json",
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    "transcriptions": "/2010-04-01/Accounts/ACaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa/Calls/CAaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa/Transcriptions.json",
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    "user_defined_message_subscriptions": "/2010-04-01/Accounts/ACaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa/Calls/CAaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa/UserDefinedMessageSubscriptions.json",
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    "user_defined_messages": "/2010-04-01/Accounts/ACaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa/Calls/CAaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa/UserDefinedMessages.json"
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  },
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  "to": "+18888888888",
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  "to_formatted": "(415) 867-5309",
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  "trunk_sid": null,
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  "uri": "/2010-04-01/Accounts/ACaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa/Calls/CAaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa.json",
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  "queue_time": "1000"
39
}

If an incoming call does not have a SHAKEN PASSporT, Twilio will sign the call with its own SHAKEN PASSporT with Attestation C with DIV signing.

Twilio will use the CallToken in the outgoing leg to verify the CallerID. If the CallerID doesn't match the CallToken, Twilio will reject the call with an error.

The following conditions must be met in order for Twilio to forward the inbound SHAKEN PASSporT along with a DIV PASSporT for the outgoing (forwarded) call:

• The CallerID of the outgoing call matches the CallerID in the CallToken of the outgoing call
• The incoming call contains a SHAKEN PASSporT
• The outbound carrier endpoint supports SHAKEN and DIV PASSporTs

The CallToken property in an incoming call webhook is a string containing a URL-encoded JSON object.

Once decoded, the CallToken will look like this:

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{
2

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   "parentCallInfoToken": "eyJhbGciOiJFUzI1NiJ9.eyJjYWxsU2lkIjoiQ0FYWFhYLi4uLiIsImZyb20iOiIrMTIyMjIyMjIyMjIiLCJ0byI6IisxNTU1NTU1NTU1NSIsImlhdCI6IjE2MzU5NTc0MjMifQ.jVOxmCbSxxKg2fuzvDT_-PTStRRw_TrWgdh2QaZNzHQwvgwO6Qk_FRPFPYguJQn19x0yZqltPQfHwql4FJt_7g",
4
   "identityHeaderTokens": [
5
       "eyJhbGciOiJFUzI1NiIsInBwdCI6InNoYWtlbiIsInR5cCI6InBhc3Nwb3J0IiwieDV1IjoiaHR0cHM6Ly9leGFtcGxlLmNvbS9zb21lQ2VydGlmaWNhdGUucGVtIn0.eyJhdHRlc3QiOiJBIiwiZGVzdCI6eyJ0biI6WyIxNTU1NTU1NTU1NSJdfSwiaWF0IjoxNjM1OTU3NDIzLCJvcmlnIjp7InRuIjoiMTIyMjIyMjIyMjIifSwib3JpZ2lkIjoic29tZS1HVUlELWhlcmUifQ.ygPO2sImJR9MSqoRVD0CvnB2euv3RUYdupNEFS3wpgecs-yi8SU8FtYkkDypn7DC-siwdPY6vvVIx39y5Nb1sg;info=<https://example.com/someCertificate.pem>;alg=ES256;ppt=\"shaken\""
6
   ]
7

8
}

parentCallInfoToken property of a CallToken

The parentCallInfoToken is a JSON Web Token (JWT). When decoded, the header and payload of the JWT will have the following shape:

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// Header
2

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   {
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     "alg": "ES256"
5
   }
6

7
// Payload
8

9
   {
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     "callSid": "CAXXXX....",
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     "from": "+12222222222",
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     "to": "+15555555555",
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     "iat": "1635957423"
14
   }
15

• The callSid value is the Call SID for the parent incoming Call Resource.
• The from value is the caller's phone number
• The to value is your Twilio number that received the call.
• The iat is a claim that specifies the timestamp for when the parent call's PASSporT was created, which is necessary for further SHAKEN/STIR validation.

identityHeaderTokens property of a CallToken

In order to verify forwarded calls, terminating service providers need the original SHAKEN PASSporT and the DIV PASSporTs for each diversion. The identityHeaderTokens property contains the information needed for Twilio to add a DIV PASSporT to the outgoing call if necessary (See Attestation section above for more information).

The identityHeaderTokens value is an array of all of the SHAKEN and DIV PASSporTs that were included as identity headers in the inbound leg of the call. If the call is signed, the array will contain one SHAKEN PASSporT, along with any DIV PASSporTs (if the call was diverted/forwarded) present on the inbound call.

Below is an example of the decoded JWT from the identityHeaderTokens array. This represents the original SHAKEN PASSporT from the incoming call.

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// Headers
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{
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   "alg": "ES256",
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   "ppt": "shaken",
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   "typ": "passport",
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   "x5u": "https://example.com/someCertificate.pem"
8
 }
9

10
// Payload
11

12
{
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   "attest": "A",
14
   "dest": {
15
     "tn": [
16
       "15555555555"
17
     ]
18
   },
19
   "iat": 1635957423,
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   "orig": {
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     "tn": "12222222222"
22
   },
23
   "origid": "some-GUID-here"
24
 }

Since there is only one string in the CallToken's identityHeaderTokens array, this means the call was not diverted before it reached your Twilio number. If the call was diverted before it reached your Twilio number, you would see a DIV PASSporT for each diversion in the identityHeaderTokens array, as well.