Sipura SPA3000 PSTN Fallback

My setup consists of a SIP phone and DECT handset with integral TAM
plugged into Line 1 of my SPA3k. These are then setup as extensions on
Asterisk (Trixbox) with a couple of VOIP provider trunks and also the
PSTN port of the SPA3k.

Incoming calls on the PSTN line call the 's' extension on Asterisk and
this then rings the DECT and SIP phones (setup as a group)

Enough background - Down to the problem :) The PSU on the switch that
the SIP phone and the Sipura are both plugged into failed leading to an
inability to receive incoming calls. I have "Auto PSTN Fallback" set to
"Yes" and assumed that this meant incoming PSTN calls would route
through to Line 1 in the event of network failure and that calls made on
Line would route through directly to the PSTN port. The failure of the
switch has highlighted the fact that my assumption is wrong - Only
outgoing calls fallover, not incoming.

I think the solution to the problem is to change the SPA3k so it rings
both Line 1 and the Asterisk s extension at the same time (taking the
DECT extension out of the ring group). I haven't sufficiently read up
on Sipura dial plans etc to work out how to do this yet :)

Anyone else had this problem and solved it?

Thanks,

Peter