[Asterisk-Users] Off Toppic-ish Telephony question

[Chris Lee]

I have a IVR system which is causing me some difficulties and am after 
some help to see if it is the source of the problems or just a victim of 
the cause.

The system is running windows 2K and it uses the the CAPI interface to 
receive calls through a FRITZ ISDN card.

Now the trouble I have is with DTMF recognition when the caller is in 
noisy environments.

If the noise exceeds a certain (as yet unknown) threshold then the 
system is not responding to DTMF.

Now my question is the following:
The users are all using Nokia GSM hand sets. As my limited knowledge 
recalls GSM sends the DTMF as signaling data and the ISDN should receive 
the DTMF without requiring operation of the audio channel. Am I correct?
If I am correct why do I have this problem;
Is it due to the nokia trying to turn the complex noise into DTMF and 
overriding the keyboard input?
Is it due to the CAPI software (at the server) getting audio channel and 
trying to get DTMF out of that and thus overriding DTMF from phone 
keyboard (If so I thought that the GSM used lower quality codec so how 
is DTMF arriving via audio channel?)

This all leads me to the situation of if it is the CAPI end that is 
causing me the problem maybe I can use asterisk and prevent audio 
channel DTMF collection (is this simple) but if it is elsewhere what can 
I do? (change handsets?)

Thanks for any insight on this

Regards
Chris Lee