1. Understand the challenges of Twilio call keeping

In Twilio, when you try to put it on hold by updating a Call Leg, the common challenge is that the other party may be disconnected due to losing the connection. This is because the update operation on a call leg usually changes its current TwiML stream, causing the original connection logic to be interrupted. For example, if you redirect the calling party to a URL that plays music, the called party may hang up because the calling party's TwiML process ends. In order to effectively manage call retention and recovery, we need to ensure that all parties involved can stay connected during call retention and can successfully reconnect when unsustained.

2. Method 1: Use conference to realize call maintenance and recovery (recommended)

Twilio's feature is the easiest and most reliable way to keep calls. It allows you to join multiple participants into a virtual meeting room and independently control the status of each participant, including putting it in a hold state.

2.1 Put participants in a hold state

When a participant joins a meeting, you can put it on hold by updating the participant's resources. This involves setting the hold parameter to True and specifying a holdUrl to play and hold music.

Implementation steps:

1. Make sure your call has entered a meeting through the verb of TwiML.
2. Get the Call SID and Conference SID of the participant you want to put in the hold state.
3. Use the Twilio client library to initiate an update request for this participant resource.

Sample Code (PHP):

 use Twilio\Rest\Client;

// Assume that you have initialized the Twilio client $sid = "ACXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
$token = "your_auth_token"; // Your Twilio Auth Token
$client = new Client($sid, $token);

// Replace with your actual Conference SID and Participant Call SID
$conferenceSid = "CFXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX; // Conference SID
$participantCallSid = "CAXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX; // Participant's call SID

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

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

In the above code, the audio file specified by holdUrl will be played looping during the participant being held.

2.2 Resuming participant calls

To restore a participant from the hold state and rejoin it to the meeting call, you simply update the participant resource again and set the hold parameter to False.

Sample Code (PHP):

 use Twilio\Rest\Client;

// Assume that you have initialized Twilio client $sid = "ACXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX;
$token = "your_auth_token";
$client = new Client($sid, $token);

// Replace with your actual Conference SID and Participant Call SID
$conferenceSid = "CFXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX;
$participantCallSid = "CAXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX;

try {
    $participant = $client
        ->conferences($conferenceSid)
        ->participants($participantCallSid)
        ->update([
            "hold" => false // Set hold to false to resume call]);

    echo "Participant" . $participant->callSid . "Recovered successfully from hold.\n";
} catch (Exception $e) {
    echo "Resuming participant call failed: " . $e->getMessage() . "\n";
}

Advantages: Use the conference function to keep calls very simple and intuitive, and Twilio automatically handles connection logic to ensure that other participants are not affected.

3. Method 2: Handle the maintenance and recovery of independent call legs (Regular Call Legs)

If you don't use the meeting function, but instead directly manage two separate call legs, it will be relatively complicated to keep the call as you need to manually handle the call flow to prevent the other party from being disconnected.

3.1 Implement call keeping

When you want to put a call leg in a hold state, you cannot simply redirect it to the hold music URL via the update operation, as this will cause the connection to the other leg to be interrupted. The correct approach is to provide a subsequent TwiML instruction after the initial TwiML so that when the caller is redirected, the called party (or other leg) can continue its call flow instead of hanging up directly.

TwiML example:

Suppose your calling party is connected to the called party via . When you need to put the caller on hold, you can update the caller's TwiML URL to point to a TwiML that plays the music. At this time, the called party (the other leg) will end its process due to changes in the caller's TwiML. In order to prevent the called party from hanging up, subsequent instructions are required to be provided to the called party in the initial TwiML.

 <!-- Initial TwiML, used to connect the calling party and the called party-->
<response>
  <dial> Called Number</dial>
  <!-- If the calling party is redirected (e.g., put on hold), the TwiML here will be effective for the called party->
  <Redirect>https://example.com/hold_destination_for_other_leg</Redirect>
</Response>

In the above TwiML, when the calling party is updated and sent to a new TwiML (for example, playing and keeping music), the verb is completed, and the called party will then continue its process according to the command instead of hanging up. https://example.com/hold_destination_for_other_leg can be a waiting command (such as plays waiting sound) or TwiML entering a queue.

3.2 Achieve call recovery

Restoring the connection of the independent call legs is more complicated because you need to re-establish the association between the two call legs. An effective way is to use Twilio's and features.

Implementation steps:

1. Hold: The call leg that needs to be held is into a queue. This queue can be a dedicated queue for retention.

    <!-- Keep the TwiML of the party and put it into the queue-->
   <response>
       <enqueue waiturl="http://www.myapp.com/hold_music.mp3">myHoldQueue</enqueue>
   </response></response>

   At the same time, the other party (not maintained) needs to be directed to a waiting state or play waiting music, waiting for the maintained party to resume.

2. When recovery: When you need to resume a call, update the unsustained call leg so that it enters the queue where the previously held party is located.

    <!-- TwiML of the unretained party, dial in the queue to connect to the maintained party -->
   <response>
       <dial>
           <queue>myHoldQueue</queue>
       </dial>
   </response>

   When the unretained party dials into the queue, Twilio will automatically connect the first member of the queue (i.e. the maintained party) to the dialed in, thereby restoring the call.

Note: This approach requires more sophisticated call status management and TwiML logic design to ensure the correct flow of calls in different scenarios.

4. Precautions and best practices

• Preferential use of conference features: For most scenarios where call persistence is required, Twilio's feature is highly recommended. It greatly simplifies call management and avoids the complexity of handling independent call legs. The conference function itself is designed for multi-party call management, including built-in functions such as keeping and muting.
• Keep music: No matter which method is used, the party being held should be provided with clear holding music or prompt sounds to improve the user experience.
• Error Handling: In your application, be sure to add an appropriate error handling mechanism to deal with the failure of Twilio API calls.
• Call process design: Before implementation, carefully plan your call process, including state transitions before, maintain, and after recovery.

Summarize

Twilio provides flexible ways to manage the retention and recovery of voice calls. For most application scenarios, leveraging and its participant management API is the easiest and most robust solution. It allows you to control the hold status of participants through simple API calls and specify the hold music. If your use case does require more underlying control of independent call legs, combining TwiML's , , and verbs can achieve complex retention and recovery logic, but this requires more detailed planning and implementation. When choosing an implementation, always weigh its complexity and maintenance costs, and prioritize simpler and more reliable meeting features.

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