docs: def preparer(telefoon):
          '''
          Edit a phone value to a value that can be validated as a
          phone number.
          This takes the incoming value and :
              Removes all whitespace ( space, tab , newline , ... ) characters
              Removes the following characters: " / - . "
              If no + is present at frond, add the country code
              In short: just add a + at the beginning of the country code.
          '''
          if telefoon is None or telefoon == colander.null:
              return colander.null
          if 'landcode' in telefoon and telefoon.get('landcode') is not None:
              landcode = telefoon.get('landcode')
              value = re.sub(r'\s+', '', landcode).replace('.', '').replace('/', '').replace(',', '').replace('-', ''). \
                  lstrip('0')
              telefoon['landcode'] = '+' + value if value[0] != '+' else value
          if 'nummer' in telefoon and telefoon.get('nummer') is not None:
              nummer = telefoon.get('nummer')
              value = re.sub(r'\s+', '', nummer).replace('.', '').replace('/', '').replace(',', '').replace('-', ''). \
                  lstrip('0')
              telefoon['nummer'] = value
          return telefoon