RMC, if you have a choice, use ATN! If not, still log on however expect very little CPDLC contact. The only time I use CPDLC to a FANS aircraft is to transfer to another cross-border agency as if the message fails, we do not need to give the spiel Juggler refers to instructing you to disconnect, we just transfer you via voice and then your CPDLC disconnects anyway to log on to the next FIR. The issue with FANS aircraft is that we have a 40 second window for the message to be read and acknowledged, however the message could still be sent to a FANS aircraft after the timeout, subsequently read and acknowledged and acted upon. The instruction could be stale and already superseded via voice, hence the spiel for you to disregard the message and to disconnect. With ATN aircraft, if the message hasn't been acknowledged after 40 seconds, it is completely deleted so it will never display in the cockpit and we can send it again if we wish.