docs: def configure(self, config=None, **kwargs):
          """
          We expect all of our config (apart from the ENGINE) to be
          in a dictionary called 'config' in our INSTALLED_BACKENDS entry
          """
          self.config = config or {}
          for key in ['messaging_token', 'number']:
              if key not in self.config:
                  msg = "Tropo backend config must set '%s'; config is %r" %\
                        (key, config)
                  raise ImproperlyConfigured(msg)
          if kwargs:
              msg = "All tropo backend config should be within the `config`"\
                  "entry of the backend dictionary"
              raise ImproperlyConfigured(msg)