Option Explicit
Private Const gueltig As String = "1234567890,"
 
Function Box(AktBox As TextBox, LetztesZeichen As Integer)
    If LetztesZeichen = vbKeyBack And Len(AktBox.Text) > 0 Then
        AktBox.Text = Mid(AktBox.Text, 1, Len(AktBox.Text) - 1)
        Exit Function
    End If
 
    If InStr(AktBox.Text, ",") <> 0 And Chr(LetztesZeichen) = "," Then
        Box = 0
        Exit Function
    End If
 
    If InStr(gueltig, Chr(LetztesZeichen)) = 0 Then
        Box = 0
    Else
        Box = LetztesZeichen
    End If
End Function

Option Explicit
Private validate As New Class1
 
 
Private Sub txtNummer_KeyPress(KeyAscii As Integer)
    KeyAscii = validate.Box(txtNummer, KeyAscii)
End Sub