You have actually figured it out yourself. You are correct that in order for the Frame Relay to work between the Cisco routers and the ATLAS you must issue the 'encapsulation frame-relay ietf'. That should resolve your problem as long as all your cross-connects are set up in the ATLAS.
Hope this hleps,
Thanks a lot! I haven't got the chance to verify this on real equipments yet but I will do it. And I just realized that I missed 'ietf' at the end of the command when I posted my thread and it was only my guessing.
Thanks for the help!