Option Explicit

Sub FormatNumberString()
  'Looks through a list of numbers on the first worksheet and formats them
  'with leading zeroes.
  
  Dim CheckRow As Long, LastRow As Long, CheckString() As String
  Dim x As Integer, arr As Long, ArrayString As Variant
  Dim NewString As String, TempString As String
  Dim wb As Workbook, ws As Worksheet
  
    Set wb = ThisWorkbook
    Set ws = wb.Worksheets(1)
    LastRow = ws.Range("T50000").End(xlUp).Row
    ReDim CheckString(0 To LastRow - 1)
    
    ws.Activate
    'Grab the values
    arr = 0
    For CheckRow = 2 To LastRow
      ws.Range("T" & CheckRow).Select 'REMOVE THIS LINE ONCE CODE WORKS
      CheckString(arr) = ws.Range("T" & CheckRow).Value
      arr = arr + 1
    Next CheckRow
    
    'Insert values back in with new format
    arr = 0
    For CheckRow = 2 To LastRow
      ws.Range("T" & CheckRow).Select 'REMOVE THIS LINE ONCE CODE WORKS
      If CheckString(arr) <> "" Then
        'Convert each number to have leading zeroes
        ArrayString = Split(CheckString(arr), ",")
        For x = 0 To UBound(ArrayString)
          Select Case Len(ArrayString(x))
            Case 1:
              TempString = "00" & ArrayString(x)
            Case 2:
              TempString = "0" & ArrayString(x)
            End Select
            NewString = CStr(NewString & TempString & ",")
          Next x
          'Remove last comma
          NewString = Mid(NewString, 1, Len(NewString) - 1)
        ws.Range("T" & CheckRow).NumberFormat = "@"
        ws.Range("T" & CheckRow).Value = NewString
        NewString = ""
      End If
      arr = arr + 1
    Next CheckRow

End Sub