Web Service not completing

I 've created an app with VS 15.0.x which is using a web service to return data.
The app is installed in my phone and its working fine.
Now i have the Visual Studio version 15.5.7 and i tried to change something to the app.
Now the app its not working. I mean the Completed event never fires.
The service is working fine and i am using it from other apps.
The code is
private void Button1_Click(object sender, System.EventArgs e)
{
bool check = NetworkInterface.GetIsNetworkAvailable();
if (check)
{
WebService client = new WebService();
client.FindPhone(txtPhone.Text );
client.FindPhoneCompleted += Client_FindPhoneCompleted;
}
else
{
lblResult.Text = "No network";
}
}

    private void Client_FindPhoneCompleted(object sender, FindPhoneCompletedEventArgs e)
    {
        string input;
        string output;
        input = e.Result;
        output = input.Replace("|", System.Environment.NewLine);

        lblResult.Text = output;
    }

The code to Client_FindPhoneCompleted is never fires
Do you have any ideas?
Thank you