public async Task AddNumbersAsync(Uri uri, int a, int b)
{
    var soapString = this.ConstructSoapRequest(a, b);
    using (var client = new HttpClient())
    {
        client.DefaultRequestHeaders.Add("SOAPAction", "http://CalculatorService/ICalculatorService/Add");
        var content = new StringContent(soapString, Encoding.UTF8, "text/xml");
        using (var response = await client.PostAsync(uri, content))
        {
            var soapResponse = await response.Content.ReadAsStringAsync();
            return this.ParseSoapResponse(soapResponse);
        }
    }
}

private string ConstructSoapRequest(int a, int b)
{
    return String.Format(@"
    
        
            
                {0}
                {1}
            
        
    ", a, b);
}

private int ParseSoapResponse(string response)
{
    var soap = XDocument.Parse(response);
    XNamespace ns = "http://CalculatorService/";
    var result = soap.Descendants(ns + "AddResponse").First().Element(ns + "AddResult").Value;
    return Int32.Parse(result);
}