Thing is (FANS issues...Shannon controllers please correct me on this one if I'm off on the wrong path!!)
If you log on with FANS (going west) then EISN can auto transfer you to EGGX. If you are on ATN then the auto transfer then doesn't occur.
Now the other week we logged on to EGTT via CPDLC on ATN and got handed over to EISN (Still ATN...). Logged off, but couldn't then log on again with them via FANS so gave up. So then we straight to EGGX. Faff, combined with lots of RT traffic on the issues about it. Also heard other aircraft with similar issues...
When EGTT accepts the FANS logon then you get to transferred seamlessly all the way to Canada without issues...
(Poor hengrish - long day!)