You could try to enter the full ddi number on the call forward unregistered on callmanager 4.2 and above. I think the setting is Forward Unregistered on the relevent phone lines so if your link fails both Pub and Sub will see the phobnes on the other...
HelloUsually a one way conversation is an IP routing problem or something stopping IP frames - in this case between the IP phone and the gateway.Can you ping the phone from the gateway? can you give a little more information regarding the problem