BT Line using a TDM400P? card with an FXO interface

The card works great besides two minor caveats:

It fails to detect call hangups from the BT side in the default BT line consumer configuration. The only way to fix the is to get a couple of time on the line to BT Faults and get them to adjust the CPC on the line to 800ms. The UK default is 120ms and TDM400P? does not pick it up as an event. BT is quite helpfull and you can have the setting changed provided that you know what are you talking about (if the person on the phone does not know, ask him politely to pass it to a more senior collegue). In UK Telco language the setting is known "Disconnect Clear Time" and is described in detail in BT SIN 351 and 352 around page 10 (351 is for 120 case, 352 is for the 800 case which also describes alternative signalling possibilities).

BT does some work in the exchanges on a nightly basis. As a result there is always a polarity change event each morning which is never followed by an actual ring. The callerid state machine in asterisk has no timeouts so it will just sit after this polarity reversal and do nothing preventing it from picking up calls. The only solution is a mandatory restart each morning after the time the polarity reversal has happened (this can be seen in logs). It is essential to try to adjust the impedance by using fxotune. Without it there will always be some residual echo which the echo canceller will end up struggling with.

The rest of the settings are as follows:

#/etc/modules
opermode=TBR21

#/etc/zaptel.conf
fxsks=4
loadzone = uk 
defaultzone = uk 
#/etc/asterisk/zapata.conf
signalling=fxs_ks
context=from-pstn 
usecallerid=yes 
echocancel=128 
echotraining=200 
echocancelwhenbridged=yes 
cidsignalling=v23 
cidstart=polarity 
sendcalleridafter=0 
callprogress=no 
language=en 
switchtype=national 
busydetect=yes 
group=1 
rxgain=8.0 
txgain=0.0 
callgroup=1 
pickupgroup=1
line="1 WCTDM/0/4 FXSKS" 
channel =>4 

-- AntonIvanov? - 13 Nov 2008