why this function not work correctc ? RasDialDlg() API ;; please any one help me

Question

hello 
how are you ?


please help me to find the error in this code

why this code work but the RasDialDlg() function not dial 

[StructLayout(LayoutKind.Sequential, CharSet=CharSet.Auto)]

public struct RASDIALDLG

{

public int size ;

public IntPtr hwndOwner;

public Int32 flags;

public Int64 xdialog;

public Int64 ydialog;

public Int32 subentry;

public Int32 error;

public IntPtr reserved;

public IntPtr reserved2;

}

[DllImport("Rasdlg.dll")]
public static extern int RasDialDlg(string phonebook, string entry,string number,
ref RASDIALDLG lpInfo);

private void button1_Click(object sender, System.EventArgs e)

{

string lpszPhonebook=null;

string lpszEntry=null;

string lpszPhoneNumber="07770500";

RASDIALDLG lpInfo = new RASDIALDLG();

int result;

lpInfo.size =Marshal.SizeOf(lpInfo);

result= RasDialDlg(lpszPhonebook,lpszEntry,lpszPhoneNumber,ref lpInfo);

}