Transfer calls

Article
02/14/2024

During an active call, you may want to transfer the call to another person or number. Let's learn how.

Prerequisites

An Azure account with an active subscription. Create an account for free.
A deployed Communication Services resource. Create a Communication Services resource.
A user access token to enable the calling client. For more information, see Create and manage access tokens.
Optional: Complete the quickstart to add voice calling to your application

Install the SDK

Use the npm install command to install the Azure Communication Services Common and Calling SDK for JavaScript:

npm install @azure/communication-common --save
npm install @azure/communication-calling --save

Initialize required objects

A CallClient instance is required for most call operations. When you create a new CallClient instance, you can configure it with custom options like a Logger instance.

With the CallClient instance, you can create a CallAgent instance by calling the createCallAgent. This method asynchronously returns a CallAgent instance object.

The createCallAgent method uses CommunicationTokenCredential as an argument. It accepts a user access token.

You can use the getDeviceManager method on the CallClient instance to access deviceManager.

const { CallClient } = require('@azure/communication-calling');
const { AzureCommunicationTokenCredential} = require('@azure/communication-common');
const { AzureLogger, setLogLevel } = require("@azure/logger");

// Set the logger's log level
setLogLevel('verbose');

// Redirect log output to console, file, buffer, REST API, or whatever location you want
AzureLogger.log = (...args) => {
    console.log(...args); // Redirect log output to console
};

const userToken = '<USER_TOKEN>';
callClient = new CallClient(options);
const tokenCredential = new AzureCommunicationTokenCredential(userToken);
const callAgent = await callClient.createCallAgent(tokenCredential, {displayName: 'optional Azure Communication Services user name'});
const deviceManager = await callClient.getDeviceManager()

How to best manage SDK connectivity to Microsoft infrastructure

The Call Agent instance helps you manage calls (to join or start calls). In order to work your calling SDK needs to connect to Microsoft infrastructure to get notifications of incoming calls and coordinate other call details. Your Call Agent has two possible states:

Connected - A Call Agent connectionStatue value of Connected means the client SDK is connected and capable of receiving notifications from Microsoft infrastructure.

Disconnected - A Call Agent connectionStatue value of Disconnected states there's an issue that is preventing the SDK it from properly connecting. Call Agent should be re-created.

invalidToken: If a token is expired or is invalid Call Agent instance disconnects with this error.
connectionIssue: If there's an issue with the client connecting to Microsoft infrascture, after many retries Call Agent exposes the connectionIssue error.

You can check if your local Call Agent is connected to Microsoft infrastructure by inspecting the current value of connectionState property. During an active call you can listen to the connectionStateChanged event to determine if Call Agent changes from Connected to Disconnected state.

const connectionState = callAgentInstance.connectionState;
console.log(connectionState); // it may return either of 'Connected' | 'Disconnected'

const connectionStateCallback = (args) => {
    console.log(args); // it will return an object with oldState and newState, each of having a value of either of 'Connected' | 'Disconnected'
    // it will also return reason, either of 'invalidToken' | 'connectionIssue'
}
callAgentInstance.on('connectionStateChanged', connectionStateCallback);

Call transfer is an extended feature of the core Call API. You first need to import calling Features from the Calling SDK:

import { Features} from "@azure/communication-calling";

Then you can get the transfer feature API object from the call instance:

const callTransferApi = call.feature(Features.Transfer);

Call transfers involve three parties:

Transferor: The person who initiates the transfer request.
Transferee: The person who is being transferred.
Transfer target: The person who is being transferred to.

Transfer to participant:

There's already a connected call between the transferor and the transferee. The transferor decides to transfer the call from the transferee to the transfer target.
The transferor calls the transfer API.
The transfer target receives an incoming call only if the transferee accepts the transfer request.

To transfer a current call, you can use the transfer API. transfer takes the optional transferCallOptions, which allows you to set a disableForwardingAndUnanswered flag:

disableForwardingAndUnanswered = false: If the transfer target doesn't answer the transfer call, the transfer follows the transfer target forwarding and unanswered settings.
disableForwardingAndUnanswered = true: If the transfer target doesn't answer the transfer call, the transfer attempt ends.

// transfer target can be an Azure Communication Services user
const id = { communicationUserId: <ACS_USER_ID> };

// call transfer API
const transfer = callTransferApi.transfer({targetParticipant: id});

Transfer to call:

There's already a connected call between the transferor and the transferee.
There's already a connected call between the transferor and the transfer target.
The transferor decides to transfer the call with the transferee to the call with transfer target.
The transferor calls the transfer API.
The transfer target receives an incoming call only if the transferee accepts the transfer request.

To transfer a current call, you can use the transfer API.

// transfer to the target call specifying the call id
const id = { targetCallId: <CALL_ID> };

// call transfer API
const transfer = callTransferApi.transfer({ targetCallId: <CALL_ID> });

The transfer API allows you to subscribe to stateChanged. It also comes with a transfer state and error properties

// transfer state
const transferState = transfer.state; // None | Transferring | Transferred | Failed

// to check the transfer failure reason
const transferError = transfer.error; // transfer error code that describes the failure if a transfer request failed

The transferee can listen to a transferAccepted event. The listener for this event has a TransferEventArgs which contains the call object of the new transfer call between the transferee and the transfer target.

// Transferee can subscribe to the transferAccepted event
callTransferApi.on('transferAccepted', args => {
    const newTransferCall =  args.targetCall;
});

The transferor can subscribe to events for change of the state of the transfer. If the call to the transferee was successfully connected with Transfer target, transferor can hang up the original call with transferee.

transfer.on('stateChanged', () => {
   if (transfer.state === 'Transferred') {
       call.hangUp();
   }
});