SendCall.php

<?php
// Ejecutar: composer require guzzlehttp/guzzle:*
require'vendor/autoload.php';

$headers=array(
'Content-Type'=>'application/x-www-form-urlencoded',
'Accept'=>'application/json',
);

$client= new \GuzzleHttp\Client();

// Define la matriz del cuerpo de la solicitud.
$request_body = array(
     "client"=>"AQUI_SU_CLIENT",
     "key"=>"AQUI_SU_KEY",
     "phone"=>"AQUI_EL_NUMERO_DE_CELULAR",
     "message"=>"AQUI_EL_MENSAJE_A_ENVIAR",
     "voice"=>"AQUI_TIPO_DE_VOZ",
     "retries"=>"AQUI_NUMERO_DE_INTENTOS",
     "leave-voicemail"=>"false",
     "audio-code"=>"AQUI_ID_AUDIO",
     ); 

try{
$response=$client->request('POST','https://www.onurix.com/api/v1/call/send',array(
'headers'=>$headers,
'form_params'=>$request_body,
)
);
print_r($response->getBody()->getContents());
}
catch(\GuzzleHttp\Exception\BadResponseException $e){
// Manejar excepciones o errores de API
print_r($e->getMessage());
}