Implementing a custom SMS transport for a SMS service is a rare event! Take a look at SMS services before implementing a new transport as the service may already have an implementation. The Fake service provides a way for testing messages in a development environment.

If you decide to implement a new SMS transport for a publicly accessible service, consider making it available as a new contrib project. Contrib projects are able to implement the required code pieces without any modifications to code owned by the SMS Framework project, as evident below.

Implementation

In a custom module, add the following boilerplate:

Services

services:
  autoconfigure: true
  autowire: true

  Drupal\my_module\MyModuleTransportFactory:
    parent: notifier.transport_factory.abstract
    tags:
      - { name: texter.transport_factory }

Factory class

namespace Drupal\my_module;

use Symfony\Component\Notifier\Transport\AbstractTransportFactory;
use Symfony\Component\Notifier\Transport\Dsn;
use Symfony\Component\Notifier\Transport\TransportInterface;

final class MyModuleTransportFactory extends AbstractTransportFactory {

  protected function getSupportedSchemes(): array {
    // Make up a "scheme" which will be used in the `sms.transports` DSN.
    return ['my-module-scheme'];
  }

  public function create(Dsn $dsn): TransportInterface {
    return new MyModuleTransport();
  }
}

Transport class

namespace Drupal\my_module;

use Symfony\Component\Notifier\Message\MessageInterface;
use Symfony\Component\Notifier\Message\SentMessage;
use Symfony\Component\Notifier\Message\SmsMessage;
use Symfony\Component\Notifier\Transport\AbstractTransport;

final readonly class MyModuleTransport extends AbstractTransport {

  protected function doSend(MessageInterface $message): SentMessage {
    assert($message instanceof SmsMessage);
    // @todo send the message here.
    // If you need a dependency from the service container, add it to the
    // transport factory then pass it down to the constructor of this class.
    // See "Dependencies" section below.
  }

  public function __toString() {
    // Return the DSN of the transport, the scheme should be the same per factory.
    return 'my-module-scheme://default';
  }

  public function supports(MessageInterface $message): bool {
    // Add other criteria if you need the message to have more specific characteristics.
    return $message instanceof SmsMessage;
  }
}

Dependencies

When a transport needs a dependency, like Guzzle, pass it down from the factory:

Using the code above as a base.

Services

No changes to services.yml are required since the above template uses autowiring.

Factory class

Add dependencies to the constructor, then pass along the dependencies to the transport.

final class MyModuleTransportFactory extends AbstractTransportFactory {
  public function __construct( 
    private \Psr\Http\Client\ClientInterface $httpClient, 
  ) {} 
  
  public function create(Dsn $dsn): TransportInterface {
    return new MyModuleTransport(
      // Pass along the dependency here:
      $this->httpClient, 
    );
  }
}

Transport class

final readonly class MyModuleTransport extends AbstractTransport {
  public function __construct( 
    private \Psr\Http\Client\ClientInterface $httpClient, 
  ) {} 

  protected function doSend(MessageInterface $message): SentMessage {
    assert($message instanceof SmsMessage);
    // @todo send the message here.
    $this->httpClient->send(...); 
  }
  
  // ...
}

---

Then configure the transport just like other transports.

Configuration

In the site directory, usually sites/default/, create a services.yml file.

Add to settings.php:

$settings['container_yamls'][] = __DIR__ . '/services.yml';

Add to services.yml:

parameters:
  sms.transports:
    mytransport: 'my-module-scheme://default'

Then clear Drupal caches:

drush cr

Continue at the Sending documentation to use the newly configured channel. Noting that the recipient you send to must be a valid SmsRecipientInterface.