function checkFields() {
  var lastname = document.getElementById('last_name');
  var mail = document.getElementById('email');
  var phone = document.getElementById('telephone');

  if (phone.value !='' && mail.value !='' && lastname.value !='') {
    return true;
  }
  else {
    alert("The following fields are mandatory:'Surname', 'Telephone', 'E-mail'");
    return false;
  }

}