I'm confused... this won't work as you set it up. To make the call go to your 1866 number you would have to pickup x123 and dial 0, not call 123.
What you want to do is go into form 9, find x123, hit more, Review, Call Forward, and set Always to SC100. There is no need to put it in a different tenant. Then dial 123 and it should forward to the 1866 number (assuming you programmed everything else as noted in the documentation for External Call Forwarding)