Unable to call using skype API in C#

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C#

WPF

Skype

I'm trying to make calls through Skype API in C#. I can only open Skype but I'm not able to make calls. I'm looking for some suggestions to solve the issue.

public void makecall()
    {
        string PhoneName = Mike;
        string MyXMLFilePath = AppDomain.CurrentDomain.BaseDirectory + @"Database\ContactsData.xml";
        XmlDocument MyXmlDoc = new XmlDocument();
        MyXmlDoc.Load(MyXMLFilePath);
        XmlNode RootNode = MyXmlDoc.SelectSingleNode("Users");
        XmlNodeList FirstLevelNodeList = RootNode.ChildNodes;
        foreach (XmlNode Node in FirstLevelNodeList)
        {
            XmlNode SecondLevelNode1 = Node.FirstChild;
            if (SecondLevelNode1.InnerText == PhoneName)
            {
                XmlNode SecondLevelNode2 = Node.ChildNodes[1];
                PhoneNumber = SecondLevelNode2.InnerText;
            }
        }
        if (PhoneNumber != null)
        {
            Call(PhoneNumber);
        }
        else
        {
            MessageBox.Show("Phone number is not available.");
        }
    }
    pubic void Call(string number)
    {
        new WebBrowser().Navigate("skype:" + number + "?call");
    }

It's the XML code I use:

<Users>
      <User>
        <Name>Mike</Name>
        <Number>1234</Number>
      </User>
    </Users>

What I have tried:

pubic void Call(string number)
{
    Call call = skype.PlaceCall(number);
    //new WebBrowser().Navigate("skype:" + number + "?call");
}

But it says "An handled exception of type 'System.Runtime.Interopservices.COMException'occured in mscorlib.dll."
Additional information: Not online

Since it says it's not oneline, then I try to Skype call via Skype API and it is able to make call.
So I'm confused.
Any suggestion?