asterisk users - Sep 2004 - DTMF signaling with GSM codec

Hi everybody,
I've got a problem concerning the DTMF detection. I have a ISDN phone 
connected to a HFC-ISDN-card using the bri-stuff-drivers. I can make 
calls via a SIP provider. In the sip.conf the DTMF mode is set to 
RFC2833. However, when I make a call using the GSM codec, the DTMF 
signals are not detected properly by Asterisk and not transmitted to the 
other party. Any ideas?

Chris