input: Write a function to check if a telephone number is valid based on specific criteria.
  assert check_telephone_number('370641') == True
  assert check_telephone_number('970641') == False
  assert check_telephone_number('390641') == True
  assert check_telephone_number('39064') == False
output: def check_telephone_number(number):
      if len(number) != 6:
          return False
      if number[0] not in ['3', '7']:
          return False
      return True