docs: def validator(node, telefoon):
          ''' A valid international phone number looks like this: + .
          It is up to 15 digits long . The country code consists of 1 to 4 digits.
          The actual number may even 15 - cells ( country code) figures cover . We're going to keep the phone numbers including a + to indicate the international call prefix.
          A valid number always begins with a + . The shortest valid number that we want to keep is: +11 ( 3 characters). The longest : +123123456789123 (16 characters).
          Additional validation (after preparation ) : If a number starts with +32 (this is a Belgian number) , it must then follow eight or nine digits . The first decimal after +32 may not be 0 .
          '''
          if telefoon.get('nummer', '') != "":
              msg = "Invalid phone number"
              landcode = telefoon.get('landcode', '')
              nummer = telefoon.get('nummer', '')
              if landcode[0] != '+' or len(landcode[1:]) > 4 or not is_number(landcode[1:]):
                  raise colander.Invalid(node,
                                         msg + ': Een geldige landcode begint met een + gevolgd door maximaal 4 cijfers')
              if landcode[0] != '+' or len(landcode[1:]) + len(nummer) > 15 or not is_number(nummer):
                  raise colander.Invalid(node,
                                         msg + ': Een geldige nummer begint met een + gevolgd door maximaal 15 cijfers')
              # validatie van Belgische telefoonnummers
              if landcode == '+32':
                  if len(nummer) not in [8, 9]:
                      raise colander.Invalid(node, msg + ': Na +32 moeten er 8 of 9 cijfers volgen')