I made the changes and applied the PMF. I can now ping the UC320W successfully from my router and other computers in the network.
I can also access the configuration from the WAN port now also which I couldn't before the ping started working. That remains a little mysterious as I would have thought the appliance would have responded despite filtering the pings.
Anyhow it all works now so I'm very pleased.
I could make and receive calls before by the way; but the lack of a response was disturbing; even though the WAN status was showing received and transmitted packets.