Raspbx unistim

I have an odd problem with Asterisk running on Raspberry Pi (Raspbx). I am using the UNISTIM channel with Nortel e1140 handset. All works perfectly apart from there is no ring or flashing ring indicator. If I pick up the phone the call is answered but ringer is not working.
I have replicated the exact same setup on another box (Intel86 setup) and it all works perfectly.

I see the ring packet being called from the CLI and the ring packet being reset when phone is hung-up but no ring.

Any thoughts?