FYI,

I have upgraded the RPTU1 firmware from 1S to 2A which has made no change. (It was the easiest route to take first)
The 280 is currently running 5E software so I guess the next stage is to put it onto the lastest known good version (6N/P from memory?)
We have performed a temp fix by diverting the branches to a virtual number which then forwards to the 280 which accepts the call so we know that the problem lies with the ETSI message received from the network.