I am also facing the same problem. While calling webservice from apex it is showing the error.

Here is the .Net Web Service code which returns just "Hello World" String:-

    public class Service1 : System.Web.Services.WebService
    {
        [WebMethod]
        public string HelloWorld()
        {
            return "Hello World";
        }
    }

Now I tried to call this using APEX as follows: -

global class HelloWorld
{

    webService static String sampleCSharpWebServiceCall()
    {
        String retVal;
        TestWebServiceForAPEX.Service1Soap csService = new TestWebServiceForAPEX.Service1Soap();
        csService.timeout_x = 90000;
        retVal = csService.HelloWorld();
        return retVal;
    }

}

Now I generated wsdl and used it in another .Net Web Application on code behind of button click I am calling APEX Method which will in turn call other WebService written in .Net

        protected void Button1_Click(object sender, EventArgs e)
        {
            try
            {
                SforceService sfs = new SforceService();
                sfs.Timeout = 90000;

                LoginResult lr = sfs.login("username", "passwordtoken");

                HelloWorldService hws = new HelloWorldService();
                hws.Timeout = 90000;
                
                hws.SessionHeaderValue = new TestWebCSharpWebService.SessionHeader();
                hws.SessionHeaderValue.sessionId = lr.sessionId;

                TextBox1.Text = hws.sampleCSharpWebServiceCall();

            }
            catch (SoapException ex)
            {
                TextBox1.Text = ex.Message;
            }

     }

But it is showing following error:-

System.CalloutException: IO Exception: Read timed out
Class.TestWebServiceForAPEX.Service1Soap.HelloWorld: line 28, column 13
Class.HelloWorld.sampleCSharpWebServiceCall: line 33, column 15
External entry point