TDM410 on UK PSTN not detecting open loop disconnect

I have a TDM410 with 4 FXO cards on a FreePBX server and cannot detect if the caller hangs during voicemail.
I have two analogue lines, one giving 500ms open-loop-disconnect and the other 800ms. There is no polarity-switching, just the voltage drops to zero. This is followed by a continuous 400Hz tone.

I have the configuration set to UK,
hanguponpolarityswitch = yes,
Enabled Busy Detect.

I cannot find any info about defining a busypattern for disconnect followed by continuous tone or any settings for minimum / maximum CPC.
As a work-arround I’ve set maximum VM message to 60 secs and require a key-press to record so I can stop going to VM if they hang up first.

I can see this is a common problem but cannot find any answers.

chan_dahdi.conf: [channels] language=en busydetect=yes busycount=3 usecallerid=yes callwaiting=yes usecallingpres=yes threewaycalling=yes transfer=yes cancallforward=yes callreturn=yes echocancel=yes echocancelwhenbridged=no echotraining=no immediate=no faxdetect=no rxgain=0.0 txgain=0.0 hanguponpolarityswitch=yes sendcalleridafter=2 cidsignalling=v23 cidstart=polarity answeronpolarityswitch=no

hanguponpolarityswitch = yes is clearly wrong. I thought there was an option for battery removal, but I cannot find it.

The tone option for disconnect supervision is a continuous tone in the UK, so I don’t think you will ever reach a busycount=3