Don't listen to what the gate agents tell you. I learned that the hard way ATL-DFW-MTY. ATL agent said I was good to go jumpseating, had me listed on both. When I got to DFW they showed me their continuity which specifically outlines which carriers they allow to international jumpseat in the cabin. Its been a while, but from my recollection it was the legacy carriers, fedex, ups and only a few others..