Hi android_oma! thanks for your fast answer, finally i could solve it!
I explain what i did:
First of all i had my drupal site in maintenance mode and that was the reason of statuscode = 503. After changing this parameter i got status code = 401.
I solved this giving permissions to everyone in people/permissions section (scroll down to "Push Notifications", and mark down whichever permissions you want) so in this way i got status code = 200 (everything ok) and I could send a push notification.
Now I´d like to develop my own "code.php" in order to send a push notification to a specific user when someone access to a public page in my site. Could you give me any clue for doing this? I´m a beginner on this and I don´t know if I must use rules, or looking at the code of push notifications module and rewrite it in my code.php, or there is another easier way..
Thanks again for your help, I doubt a lot that I had achieved it without your tutorial.