def get_signature_callback_url(self, signature):
        """Return URL where backend can post signature notifications.

        Raise ``NotImplementedError`` in case the backend does not support
        "signature callback url" feature.

        Default implementation reverses :meth:`get_signature_callback_url_name`
        with ``signature.pk`` as argument.

        """
        return reverse(
            self.get_signature_callback_url_name(),
            args=[signature.pk])