To answer my own question, and provide a solution, should anyone else encounter similar issues...
The VOIP box has been 'stuck' with a default message telling me it is 'defining network settings' for over a week. Using some network monitoring software I could see it was trying, every few seconds, to connect to the DHCP server on the router but was failing.
I connected it back to the old router and had the same problem, I was unable to access anything as it was failing to get an IP address allocated.
I've spent a lot of time pressing buttons and eventually found a method to get access. Pick up the phone, listen to the message, press hash, hear the message again, press hash again - I was then invited to 'choose mode' - nothing about this in the manual! Press 3 and I was asked to 'enter IP address' and after a bit of trial and error I worked out it needed to be told what IP address to connect itself to so I gave it 192.168.0.8, the next available on the router. This has to be entered as 192*168*0*8*, yes, a trailing star to set it.
Imagine how many times I went round that loop before finding the correct way to do it !
To my amazement, I was then able to bring up a management web page using the IP address I had given it - at last!
My VOIP services are now back up and running, via the new router.
Maybe someone else who has the same device will benefit from this information...