asterisk users - May 2005 - TDM400P does not detect hangup on UK BT analogue line

Dear Collective ...

I know that this problem crops up again and again, but I've yet to find
something that works for me. I've completely exhausted Google.

I have a TDM400P card with a single FXO module connected to a standard
analogue BT telephone line. The card works fine, there are no IRQ issues or
crackling or echoing or any of that rubbish.

However, the card and/or Asterisk fail to detect when the remote party have
hungup. This isn't a problem if it's a real telephone conversation with
a
user of the system as in such cases Asterisk detects the hangup from the
local end and terminates the channel, but if someone calls in and gets to an
un-manned service such as an IVR menu or voicemail and then hangs up, the
channel remains open and you get the "2 minutes of continous tone"
voicemail
problem.

I have correctly modprobed wcfxs with "UK" as the opermode (dmesg
confirms):

Zapata Telephony Interface Registered on major 196
PCI: setting IRQ 11 as level-triggered
PCI: Assigned IRQ 11 for device 0000:00:08.0
Freshmaker version: 71
Freshmaker passed register test
Module 0: Not installed
Module 1: Not installed
Module 2: Not installed
Module 3: Installed -- AUTO FXO (UK mode)
Found a Wildcard TDM: Wildcard TDM400P REV E/F (4 modules)
Registered tone zone 4 (United Kingdom)

This is my /etc/zaptel.conf:

loadzone=uk
defaultzone=uk
fxsks=4

This is my /etc/asterisk/zapata.conf:

[channels]

language=en
context=default
signalling=fxs_ks

usecallerid=yes
cidsignalling=v23
cidstart=polarity
hidecallerid=no
restrictcid=no

callwaiting=no
echocancel=yes
busydetect=yes
callprogress=yes
hanguponpolarityswitch=yes

echocancel=yes
rxgain=4.5
txgain=4.5

immediate=no
context=incoming
channel => 4

All the polarity-based caller-ID stuff works, so I know that the card is
capable of detecting polarity switches, but the (ideal looking)
"hanguponpolarityswitch" parameter has no effect.

What am I missing? Surely this can't be a bug!

Thanks

Stuart