This article will explore in-depth how to use Twilio to achieve the holding and unholding functions of telephone calls. We will detail two main approaches: efficient management with Twilio Conference features and how to handle independent call legs with exquisite TwiML logic when meetings are not used. By comparing these two methods and providing corresponding code examples, we aim to help developers choose the solution that best suits their scenarios, ensuring smooth call flow and optimization of user experience.

1. Overview of Twilio call keeping mechanisms

In Twilio, implementing call hold function means temporarily separating one participant from the main call, usually playing and keeping music while keeping the call connected. When unhold is required, the participant is reconnected back to the main call. Updating the URL of the call leg directly may cause the other party to hang up because the default behavior of Twilio is to end the current call leg after the TwiML is executed. Therefore, we need a more granular strategy to manage call status.

2. Recommended plan: Use Twilio Conference to manage call keeping

The Twilio meeting feature is the easiest and most robust way to keep calls and cancel. By joining call participants to the meeting, you can easily manage the status of each participant, including putting it on a hold state.

2.1 Put participants in a hold state

When a participant is already in a meeting, you can put it on hold by updating that participant resource. This requires setting the hold parameter to True and specifying holdUrl to play and hold music.

 <?php // Introduce Twilio SDK
require __DIR__ . &#39;/vendor/autoload.php&#39;;
use Twilio\Rest\Client;

// Your Twilio Account SID and Auth Token
$sid = "ACXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX; // Replace with your Account SID
$token = "your_auth_token"; // Replace with your Auth Token
$client = new Client($sid, $token);

// Suppose you already have a conference SID and a participant SID
$conferenceSid = "CFXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX; // Replace with your meeting SID
$participantSid = "CAXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX; // Replace with the participant SID to be maintained

try {
    $participant = $client
      ->conferences($conferenceSid)
      ->participants($participantSid)
      ->update([
        "hold" => true,
        "holdUrl" => "http://www.myapp.com/hold_music.mp3" // Keep the URL of the music
      ]);

    echo "Participant" . $participant->callSid . " Successfully placed in hold state.\n";
} catch (Exception $e) {
    echo "Update participant status failed: " . $e->getMessage() . "\n";
}
?>

In the above code:

• $conferenceSid is the unique identifier of the meeting where the current call is located.
• $participantSid is a unique identifier for the specific participant you want to put in the hold state.
• The hold parameter is set to true to hold the participant.
• holdUrl Specifies the URL of the audio file to be played when the participant is in a hold state. This URL can be a static audio file or a dynamic URL that returns a TwiML or directive.

2.2 Cancel participants to remain in place

To cancel the hold status of a participant, simply update the participant resource again and set the hold parameter to False.

 <?php // Introduce Twilio SDK
require __DIR__ . &#39;/vendor/autoload.php&#39;;
use Twilio\Rest\Client;

// Your Twilio Account SID and Auth Token
$sid = "ACXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX; // Replace with your Account SID
$token = "your_auth_token"; // Replace with your Auth Token
$client = new Client($sid, $token);

// Suppose you already have a conference SID and a participant SID
$conferenceSid = "CFXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX; // Replace with your meeting SID
$participantSid = "CAXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX; // Replace with the participant SID to be unstolen

try {
    $participant = $client
      ->conferences($conferenceSid)
      ->participants($participantSid)
      ->update([
        "hold" => false // Set hold to false to cancel the hold]);

    echo "Participant" . $participant->callSid . "Retained successfully, return to the meeting.\n";
} catch (Exception $e) {
    echo "Update participant status failed: " . $e->getMessage() . "\n";
}
?>

2.3 Advantages of conference plans

• Simplified management: Twilio automatically handles connections and state transitions between meeting participants.
• Flexible Control: The retention status of each participant in the meeting can be independently controlled.
• Strong scalability: easy to expand to multi-party call scenarios.

3. Non-conference program: Manage independent call legs (Call Legs)

If you choose not to use the meeting feature, but instead directly manage two separate call legs, it will be much more complicated to keep the call. This is because when you update the TwiML command for one call leg, Twilio will assume that the previous TwiML command has been completed and may hang up the other leg.

3.1 Challenges and countermeasures for maintaining the call legs

The phenomenon mentioned in the original question—updating the TwiML of the primary number causes the secondary number to disconnect—is an embodiment of this complexity. To avoid this, when you "keep" one of the calls legs, you need to provide subsequent TwiML instructions for the other party to prevent them from hanging up.

For example, the initial TwiML might be as follows:

 <response>
  <dial> CUSTOMER_NUMBER</dial>
  <!-- After dialing, if the other leg is transferred, this leg will continue to execute the following TwiML -->
  <redirect>https://example.com/hold_for_agent</redirect>
</response>

When you update one of the legs through the API (for example, transferring the agent's legs to the keep music URL), the client's legs do not hang up after is finished, but instead execute the directive to direct it to a new TwiML URL (for example, a page that plays and waits for music).

3.2 Implementing complex logic of keeping and canceling keeping

In this non-conference mode, implementing hold and canceling hold requires the following steps:

1. Keep it in action:

   • Update the TwiML of the call leg that needs to be kept to point to a URL that plays the music that keeps you.
   • Meanwhile, make sure that the TwiML of the other leg does not hang up directly after its dialing is complete, but redirects to a TwiML in a "wait" state (e.g., play waiting for music or enter a queue).
   • Alternatively, a more recommended way is to enqueue the call leg that needs to be kept into a Twilio queue.

2. Cancel the hold operation:

   • When you need to unstick, you need to dial the other leg of the previously "waiting" state into the queue. Twilio will automatically connect the waiting calls in the queue to the calls in the dial-in queue.

Example TwiML (Entertainment):

When you want to keep the agent on call, you can update the agent's TwiML:

 <!-- This TwiML will put the agent into a queue called "support_queue" -->
<response>
  <enqueue waiturl="http://your-app.com/wait_music.mp3">support_queue</enqueue>
</response>

Example TwiML (Dial Enqueue Cancel Hold):

When you want to unstay and reconnect agents and clients, you can update your client's TwiML:

 <!-- This TwiML will dial the client into "support_queue" and connect to the agent in the queue -->
<response>
  <dial><queue>support_queue</queue></dial>
</response>

This approach requires you to accurately manage the TwiML state and logic flow of each call leg, and a backend service is required to dynamically generate and update TwiML.

4. Summary and best practices

• It is highly recommended to use the Twilio Conference feature to enable call hold and cancel hold. It provides a built-in, easy-to-use API to manage participant status, greatly simplifying development complexity.
• If you cannot use the meeting for a specific reason, managing independent call legs requires more complex TwiML logic and backend coordination. This includes using to control call flow, and using and to achieve "keep" and "reconnect" of calls.
• Always consider user experience when designing a call process. Make sure that the appropriate music or prompt sound is played during call hold and provides a smooth transition when the hold is cancelled.

By choosing the right approach and carefully designing the TwiML process, you can provide your users with a seamless and professional call-keeping experience.

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