Excel - Data Validation list from filtered table

In sheet Customers, pick some cell and enter:

=SUBTOTAL(103,A:A)

This formula will be re-calculated every time the filter is changed for column A.

In the Customers worksheet code area, install the following event macro:

Private Sub Worksheet_Calculate()
    Call makeDV
End Sub

In a standard module, install the following code:

Public DVList As String

Public Sub makeDV()

    Dim A As Range, r As Range
    Dim c As Collection, v As Variant
    Set c = New Collection
    Set A = Intersect(Range("A2:A" & Rows.Count), ActiveSheet.UsedRange).Cells.SpecialCells(xlCellTypeVisible)
    DVList = ""

    On Error Resume Next

    For Each r In A
        v = r.Value
        If v <> "" Then
           c.Add v, CStr(v)
           If Err.Number = 0 Then
               DVList = DVList & "," & v
           Else
            Err.Clear
           End If
        End If
    Next r

    If Len(DVList) <> 0 Then DVList = Mid(DVList, 2)
    On Error GoTo 0

    Dim Quote As Worksheet
    Set Quote = Sheets("Quote")

    With Quote.Range("C13").Validation
        .Delete
        .Add Type:=xlValidateList, AlertStyle:=xlValidAlertStop, Operator:= _
        xlBetween, Formula1:=DVList
        .IgnoreBlank = True
        .InCellDropdown = True
        .InputTitle = ""
        .ErrorTitle = ""
        .InputMessage = ""
        .ErrorMessage = ""
        .ShowInput = True
        .ShowError = True
    End With
End Sub

Whenever the filter is modified on sheet Customers, the data validation for cell C13 on sheet Quotes will up updated.