ConfigSIP readIn = new ConfigSIP();
readIn.ReadInConfig();
string sip_ip = readIn.sip_ip(); // Get nothing here.

public class ConfigSIP
{
    private string _sip_ip;

        // Fill the data table and assign the sip ip.
    public void ReadInConfig()
    {
	    DataTable dt = new DataTable("Admin");
	    dt.ReadXmlSchema(@"C:Config.xml");
	    dt.ReadXml(@"C:Config.xml");

	    _sip_ip = dt.Rows[0]["Sip_ip"].ToString();
    }

        // Return the sip ip address.
    public string sip_ip()
    {
	    return _sip_ip;
    }
}

ConfigSIP readIn = new ConfigSIP();
readIn.ReadInConfig();
string sip_ip = readIn.sip_ip();