How not to save details in text file if validation fails using VB?

Public Class Form1
    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        ' Button to validate Phone numbers typed into TextBoxes
        Dim emptyTextBoxes =
                            From txt In Me.Controls.OfType(Of TextBox)()
                            Where txt.Text.Length = 0
                            Select txt.Name

        ' An array of type String() as equivalent of your Phones in TXT boxes
        Dim txtBoxes =
                        From txt In Me.Controls.OfType(Of TextBox).Reverse.ToArray
                        Select txt.Text

        Dim Result =
                     From txt In txtBoxes.Select(Function(Phone, Index) _
                         New With
                           {
                               .Phone = Phone,
                               .Index = Index,
                               .Length = Phone.Length
                           })
                     Order By txt.Index Ascending

        Dim i, j As Integer
        Dim WrongTxtBoxesA As New System.Text.StringBuilder
        Dim WrongTxtBoxesB As New System.Text.StringBuilder
        Dim Customers As New System.Text.StringBuilder
        For Each box In Result
            ' Presume that correct phone number IsNumeric and 9 digits long, otherwise is WRONG
            If Not (IsNumeric(Result(i).Phone.ToString)) Or Result(i).Phone.Length <> 9 Then
                WrongTxtBoxesA.Append(String.Format("The Box {0} is WRONG {1} {2}", Result(i).Index, Result(i).Phone, vbCrLf))
                WrongTxtBoxesB.Append(i.ToString & Space(1))
            Else
                Customers.Append("CustomerID:" & i.ToString & vbTab & "Phone:" & vbTab & Result(i).Phone & vbCrLf)
                j += 1
            End If
            i += 1
        Next

        If i > 0 Then
            MsgBox(String.Format("{0} {1} Please update following textboxes: {2}", WrongTxtBoxesA.ToString, vbCrLf, WrongTxtBoxesB.ToString))
        End If

        If j = 0 Then
            MsgBox("NO  customer has been successfully registered!!!")
        Else
            MsgBox(String.Format("{0} {1} {2} customer has been successfully registered ", Customers.ToString, vbCrLf, j))

            Dim Filenum As Integer = FreeFile()
            FileOpen(Filenum, "Z:\Desktop\Customers.txt", OpenMode.Append)
            'Text file is opened'

            PrintLine(Filenum, Customers.ToString)
            'The data entered in the above text boxes is combined together separated with commas and stored into the open text file'

            FileClose(Filenum)
            'Once this is done the text file is closed and message below is displayed'
        End If

    End Sub

    Private Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click
        ' Button to populate 7 TxtBoxes and 7 labels
        Dim Boxes As String() = {"1234567xx", "123456xxx", "", "123456789", "987654321", "0", "PhoneNumber"}

        Dim txtboxes() As TextBox = Me.Controls.OfType(Of TextBox).Reverse.ToArray
        Dim labels() As Label = Me.Controls.OfType(Of Label).Reverse.ToArray

        Dim i As Integer
        For Each box In txtboxes
            box.Text = Boxes(i)
            i += 1
        Next
        i = 0
        For Each label In labels
            label.Text = "Box" & String.Format(i)
            i += 1
        Next
    End Sub
End Class