Server did not recognize the value of HTTP Header SOAPAction

I searched the web and experts-exchange for this problem and none of the solutions given seems to work in my case.
I am using a VB.NET Client with an ASP.NET Web Service.
The VB.NET client code to send the XML request is as follows........

        Dim xmlHTTP As MSXML2.XMLHTTP40 = New MSXML2.XMLHTTP40
        Dim xmlDoc As MSXML2.DOMDocument = New MSXML2.DOMDocument
        Dim url As String = "http://localhost/VoicesService/Service1.asmx"
        Dim strRequest As String
        Dim a As Integer = 2
        Dim b As Integer = 3

        strRequest = "<?xml version = '1.0' ?>" & _
        "<soap:Envelope " & _
        "xmlns:xsi='http://www.w3.org/2001/XMLSchema-instance' " & _
        "xmlns:xsd='http://www.w3.org/2001/XMLSchema' " & _
        "xmlns:soap='http://schemas.xmlsoap.org/soap/envelope/'> " & _
        "<soap:Body>" & _
        "<Add xmlns='http://tempuri.org/'>" & _
        "<a>" & a & "</a>" & _
        "<b>" & b & "</b>" & _
        "</Add></soap:Body></soap:Envelope>"
 
        xmlHTTP.open("POST", url, False)
        xmlHTTP.setRequestHeader("SOAPAction", "http://tempuri.org/Add")
        xmlHTTP.setRequestHeader("Content-Type", "text/xml; charset=UTF-8")
        xmlHTTP.setRequestHeader("Content-Length", "" & strRequest.Length)

        If xmlDoc.loadXML(strRequest) Then
            xmlHTTP.send(xmlDoc.xml)
            txtDisplay.AppendText(xmlHTTP.responseText)
            txtDisplay.AppendText(vbCrLf)
        Else
            txtDisplay.AppendText(xmlDoc.parseError.reason)
            txtDisplay.AppendText(xmlDoc.parseError.line)
        End If

        xmlHTTP = Nothing

...................................................................................
And the Web Service code is

Imports System.Data.Odbc
Imports System.Web.Services
Imports System.Web.Services.Protocols
Imports System.Web.Services.Description

<System.Web.Services.WebService(Namespace:="http://tempuri.org/VoicesService/Service1"), _
SoapDocumentService(SoapBindingUse.Literal, _
SoapParameterStyle.Wrapped, _
RoutingStyle:=SoapServiceRoutingStyle.SoapAction)> _
Public Class Service1

    Inherits System.Web.Services.WebService

    <WebMethod()> _
        Public Function Add(ByVal a As Integer, ByVal b As Integer)
        Return a + b
    End Function

End Class

Any sugesstions why the SOAPAction is not recongnized by the server.

Regards,
Cannibal CorpseStart Free Trial