Execute Function When Cell Value Changes

I'm doing a function to comprobate that a cell value has 11 digits. I'm not use only a validation because the the cell value can start with a 0. So I make this function

'This function is to get all the characters from a cell
Function ValidaSeguro(SS As String) As Boolean
Dim strArray() As String
Dim lLoop As Long, lCount As Long
Dim Respuesta As Integer
lCount = Len(SS)
If lCount <> 11 Then
ValidaSeguro = False
Respuesta = MsgBox("Don't have 11 digits")
Exit Function
End If
Redim strArray(lCount - 1)
For lLoop = 0 To lCount - 1.........................