Dialogflow telephony transfer call using webhook

Question

I want to integrate dialogflow transfer call using webhook.

I tried with this

const response = {
  fulfillmentMessages: [
    {
      platform: "TELEPHONY",
      TelephonyTransferCall: {
        phone_number: "+1*************",
      },
    },
  ],
};

agent.add(
  new Payload("GOOGLE_TELEPHONY", response, {
    rawPayload: true,
    sendAsMessage: true,
  })
);

but its not working. Can someone help me resolve this?