asterisk users - Dec 2004 - Asterisk cannot read DTMF based CallerID from PSTN

Hello,

I am trying to make my Asterisk to recognize CallerID from incoming
PSTN calls. I am using a TDM400. CallerID in my country's PSTN is
based on DTMF. Some information from my configuration files:

/etc/zaptel.conf:
loadzone=nl
defaultzone=nl

/etc/asterisk/zapata.conf
usecallerid=yes
cidsignalling=dtmf
cidstart=polarity

As displayed on the console, my Asterisk cannot detect the event
PreRing, but only the event Ring

Nov 27 21:00:57 NOTICE[28591]: Got event 2 (Ring/Answered)...

As I understand, in DTMF, the caller id information is sent before the
first ring. If Asterisk cannot detect the PreRing event, it cannot
recognize the caller id

I use ztmonitor to record the voice on the channel, then use SoX to
convert the file to a WAV file, then I can here the tone of the caller
id sent before the first ring.

Can anyone answer me why this problem happens?

Thanks
Hoa Nguyen