def purchase(self, status_callback_url=None, **kwargs):
        """
        Attempt to purchase the specified number. The only required parameters
        are **either** phone_number or area_code

        :returns: Returns a :class:`PhoneNumber` instance on success,
                  :data:`False` on failure
        :raises: A :exc:`TypeError` if neither phone_number or area_code
        is specified.
        """
        kwargs["StatusCallback"] = kwargs.get("status_callback",
                                              status_callback_url)

        if 'phone_number' not in kwargs and 'area_code' not in kwargs:
            raise TypeError("phone_number or area_code is required")

        number_type = kwargs.pop('type', False)
        uri = self.uri
        if number_type:
            uri = "%s/%s" % (self.uri, TYPES[number_type])

        params = transform_params(kwargs)
        resp, instance = self.request('POST', uri, data=params)

        return self.load_instance(instance)

    def iter(self, **kwargs):
        """ Return all instance resources using an iterator

        This will fetch a page of resources from the API and yield them in
        turn. When the page is exhausted, this will make a request to the API
        to retrieve the next page. Hence you may notice a pattern - the library
        will loop through 50 objects very quickly, but there will be a delay
        retrieving the 51st as the library must make another request to the API
        for resources.

        Example usage:

        .. code-block:: python

            for message in client.messages:
                print message.sid
        """
        params = transform_params(kwargs)

        while True:
            resp, page = self.request("GET", self.uri, params=params)

            if self.key not in page:
                raise StopIteration()

            for ir in page[self.key]:
                yield self.load_instance(ir)

            if not page.get('next_page_uri', ''):
                raise StopIteration()

            o = urlparse(page['next_page_uri'])
            params.update(parse_qs(o.query))

 def update(self, **kwargs):
     """
     Update your Twilio Sandbox
     """
     resp, entry = self.request("POST", self.uri,
                                body=transform_params(kwargs))
     return self.create_instance(entry)

    def update_instance(self, sid, body):
        """
        Update an InstanceResource via a POST

        sid: string -- String identifier for the list resource
        body: dictionary -- Dict of items to POST
        """
        uri = "%s/%s" % (self.uri, sid)
        resp, entry = self.request("POST", uri, data=transform_params(body))
        return self.load_instance(entry)

    def create_instance(self, body):
        """
        Create an InstanceResource via a POST to the List Resource

        :param dict body: Dictionary of POST data
        """
        resp, instance = self.request("POST", self.uri, data=transform_params(body))

        if resp.status_code not in (200, 201):
            raise TwilioRestException(resp.status_code, self.uri, "Resource not created")

        return self.load_instance(instance)

    def list(self, type="local", country="US", region=None, postal_code=None,
             lata=None, rate_center=None, **kwargs):
        """
        Search for phone numbers
        """
        kwargs["in_region"] = kwargs.get("in_region", region)
        kwargs["in_postal_code"] = kwargs.get("in_postal_code", postal_code)
        kwargs["in_lata"] = kwargs.get("in_lata", lata)
        kwargs["in_rate_center"] = kwargs.get("in_rate_center", rate_center)
        params = transform_params(kwargs)

        uri = "%s/%s/%s" % (self.uri, country, self.types[type])
        resp, page = self.request("GET", uri, params=params)

        return [self.load_instance(i) for i in page[self.key]]

    def list(self, type=None, **kwargs):
        """
        :param phone_number: Show phone numbers that match this pattern.
        :param friendly_name: Show phone numbers with this friendly name
        :param type: Filter numbers by type. Available types are
            'local', 'mobile', or 'tollfree'

        You can specify partial numbers and use '*' as a wildcard.
        """

        uri = self.uri
        if type:
            uri = "%s/%s" % (self.uri, TYPES[type])

        params = transform_params(kwargs)
        resp, page = self.request("GET", uri, params=params)

        return [self.load_instance(i) for i in page[self.key]]

    def iter(self, **kwargs):
        """
        Return all instance resources using an iterator
        """
        params = transform_params(kwargs)

        while True:
            resp, page = self.request("GET", self.uri, params=params)

            if self.key not in page:
                raise StopIteration()

            for ir in page[self.key]:
                yield self.load_instance(ir)

            if not page.get('next_page_uri', ''):
                raise StopIteration()

            o = urlparse(page['next_page_uri'])
            params.update(parse_qs(o.query))

    def get_instances(self, params):
        """
        Query the list resource for a list of InstanceResources.

        Raises a :exc:`~twilio.TwilioRestException` if requesting a page of
        results that does not exist.

        :param dict params: List of URL parameters to be included in request
        :param int page: The page of results to retrieve (most recent at 0)
        :param int page_size: The number of results to be returned.

        :returns: -- the list of resources
        """
        params = transform_params(params)

        resp, page = self.request("GET", self.uri, params=params)

        if self.key not in page:
            raise TwilioException("Key %s not present in response" % self.key)

        return [self.load_instance(ir) for ir in page[self.key]]