function hasValidPrefix()
{
  var msisdn = document.getElementById("msisdn").value;
  var prefix_1  = msisdn.substr(0,5);

  if (prefix_1 != "62852" && prefix_1 != "62853" && prefix_1 != "62813" && prefix_1 != "62812" && prefix_1 != "62811" && prefix_1 != "62856" && prefix_1 != "62858" && prefix_1 != "62815" && prefix_1 != "62816" && prefix_1 != "62855" && prefix_1 != "62857" && prefix_1 != "62859" && prefix_1 != "62817" && prefix_1 != "62818" && prefix_1 != "62819" && prefix_1 != "62877" && prefix_1 != "62878" && prefix_1 != "62817")
  {
    alert("Nomor anda tidak dikenal. Harap masukkan nomor GSM anda.");
    return false;
  }
  return true;
}