A400p Ghost / False rings

I have recently installed a couple of Asterisk servers using the A400p card connected to a standard analogue telephone line in the UK.

Everything works perfectly except a few times a day a ghost or false call rings, there is no caller ID - it just shows as Asterisk.
The Asterisk server is not making the call its just a false ring, it follows through the IVR that is setups and follows the default route until the call is answered, when the call is answered it is completely dead.

All the settings appear to be correctly setup, there is no delay when a call comes in as some people have reported, and the calls hangup correctly without any problems. It's setup to respond to polarity switching and the caller ID works everytime with the correct signalling set.

Does anyone have any idea what is causing the false ringing?

Hi,
Please show me you configuration file /etc/asterisk/chan_dahdi.conf .

[channels]
;context=from-pstn
signalling=fxs_ks
rxwink=300 ; Atlas seems to use long (250ms) winks
usecallerid=yes
ukcallerid=yes
cidsignalling=v23
cidstart=polarity
sendcalleridafter=1
;answeronpolarityswitch=yes
hanguponpolarityswitch=yes
callerid=asreceived
hidecallerid=no
callwaiting=yes
usecallingpres=yes
callwaitingcallerid=yes
threewaycalling=yes
transfer=yes
canpark=yes
cancallforward=yes
callreturn=yes
echocancel=yes
echocancelwhenbridged=no
faxdetect=incoming
echotraining=800
rxgain=0.0
txgain=0.0
;callgroup=1
;pickupgroup=1

The sample config file still has the following turned on
callwaitingcallerid=yes
threewaycalling=yes
transfer=yes
canpark=yes
cancallforward=yes
callreturn=yes
echocancelwhenbridged=yes
callgroup=1
pickupgroup=1

1. Please show us the log information when the false ring happened.
2. The version of asterisk and dahdi?

I'm going to have a problem getting the logs as these 2 machines are already with clients.

Both of these machines are doing the same thing.

1) Elastrix 2.0.3
2) Asterisk 1.6.2.19 - Dahdi 2.4.1.2

Both machines have totally different dial plans.

any feedback on the false rings?