Retaining listbox items even after form is closed in C#, WPF

I have a listbox in which i am adding items from the textbox by clicking a button. What i want is even when i close the form every time when i open the form the list should be displayed which is present or saved earlier in the listbox.

I am unable to display the list in the listbox when the form is closed.

private void Add_PhoneNumber_Click(object sender, RoutedEventArgs e)
        {
               string Country_Code = "+91";
               ListBox_PhnNumbr.Items.Add(Country_Code + Phone);
               MessageBox.Show(String.Format("Phone Number : {0}{1} is Added to List", Country_Code, Phone));
        }

SAVE Button :
<pre>private SOTA_whitelist_Number RetrieveSOTAConfiguration()
        {
              string Number1 = string.Empty;
              string Number2 = string.Empty; 
              string Number3 = string.Empty; 
              string Number4 = string.Empty; 
              string Number5 = string.Empty; 

              if (ListBox_PhnNumbr.Items.Count > 0) Number1 = ListBox_PhnNumbr.Items[0].ToString();
              if (ListBox_PhnNumbr.Items.Count > 1) Number2 = ListBox_PhnNumbr.Items[1].ToString();
              if (ListBox_PhnNumbr.Items.Count > 2) Number3 = ListBox_PhnNumbr.Items[2].ToString();
              if (ListBox_PhnNumbr.Items.Count > 3) Number4 = ListBox_PhnNumbr.Items[3].ToString();
              if (ListBox_PhnNumbr.Items.Count > 4) Number5 = ListBox_PhnNumbr.Items[4].ToString();

              MessageBox.Show("SOTA Configuration Saved Successfully"); 
              return new SOTA_whitelist_Number(Number1, Number2, Number3, Number4, Number5);
    }