asterisk users - Sep 2007 - "HiarPinning" via TDM400 in the UK ...

So I have an application where the users want to divert incoming calls on 
one analogue line out to another analogue line - both lines are supplied 
by BT and theres a TDM400 in the box.

Call comes in, system Dial's the forwarding number and bridges the calls.

Works fine.

Until one (or both) leg hangs up.

And the box never sees the hangup.

So it sits there with both Zap channels open until it's restarted.

Is this to be expected, with it just being a feature of not reliably being 
able to tell when an analogue line hangs up?

Or is there something I can do to try to detect the hangup...

The zap configuration files are all as "normal" as I've used in
other
boxes:

zaptel.conf:
   fxsks=3
   fxsks=4
   loadzone=uk
   defaultzone=uk


zapata.conf:

   [channels]

   usecallerid=yes
   cidsignalling=v23
   cidstart=polarity
   hidecallerid=no
   callwaiting=no
   threewaycalling=yes
   transfer=yes
   echocancel=yes
   echotraining=yes
   echocancelwhenbridged=yes
   immediate=no
   faxdetect=no

   ; Channel 3: PSTN line
   context=incoming
   group=1
   usecallerid=yes
   faxdetect=none
   signalling=fxs_ks
   rxgain=6
   txgain=6
   callerid=asreceived
   channel => 3

   ; Channel 4: PSTN line
   context=incoming
   group=1
   usecallerid=yes
   faxdetect=none
   signalling=fxs_ks
   rxgain=6
   txgain=6
   callerid=asreceived
   channel => 4

The extension.conf bit that does the forwarding is nothing more than:

   exten => incoming,n,Dial(Zap/G1/9${FWD})

The "9" is there because it's a BT "featureline" and
they need to dial 9
on the line itself. (BT insisted on it to create a 2-line hunt group for 
their incoming calls)

They're shortly going to move to using an Internet trunk to place outgoing 
calls on, which will very probably eliminate this issue (I'd hope), but is 
there any hope for an analogue only system doing this correctly?

Thanks,

Gordon