php curl error 403

I am trying to make an https post using curl in php and get the following error

URL: /submit.php?function=answer&data=true

Curl failed: Received HTTP code 403 from proxy after CONNECT

Here's my code

<?php
  $spark_url = 'https://api.spark.io/v1/devices/xxxxx/';
  $token     = 'xxxxx';
  $function  = urlencode($_REQUEST['function']);
  $data      = urlencode($_REQUEST['data']);

  // Initialize curl
  $ch = curl_init();

  //set the url, number of POST vars, POST data
  curl_setopt($ch,CURLOPT_URL,$spark_url.$function);
  curl_setopt($ch,CURLOPT_POST,2);
  curl_setopt($ch,CURLOPT_POSTFIELDS,'access_token='.$token.'&params='.$data);
  curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, 0);

  //execute post
  $result = curl_exec($ch);
  if ($result === FALSE) {
    die("Curl failed: " . curl_error($ch));
  }
  // Results of post in $result
  echo $result;
?>

It worked fine on my WAMP server. Any idea what's going on?

[Gianluca]

You should go in your AlterVista control panel and enable server to server connection to all the destinations.

That did it, thanks!