code: def is_valid_callsign(self, callsign, timestamp=timestamp_now):
          
          try:
              if self.get_all(callsign, timestamp):
                  return True
          except KeyError:
              return False
docs: Checks if a callsign is valid
  
          Args:
              callsign (str): Amateur Radio callsign
              timestamp (datetime, optional): datetime in UTC (tzinfo=pytz.UTC)
  
          Returns:
              bool: True / False
  
          Example:
              The following checks if "DH1TW" is a valid callsign
  
              >>> from pyhamtools import LookupLib, Callinfo
              >>> my_lookuplib = LookupLib(lookuptype="countryfile")
              >>> cic = Callinfo(my_lookuplib)
              >>> cic.is_valid_callsign("DH1TW")
              True