asterisk users - Jun 2006 - DTMF Detection: Where it happens actually?

Hello,
Could anyone help me to figure out the following questions, please:
1. Whenever there is an incoming DTMF signal on the Zap channel, where 
does the processing actually take place: In Asterisk?; or in Zaptel Drivers?
2. I'm having a problem of double (or sometimes tripple) detection of a 
single DTMF received whenever I'm calling in through a Mobile Phone. I 
guess I need to make some ammendments in the DTMF detection algorithm. 
Now the question is where can I find the code responsible for DTMF 
processing: in channel driver?; or in the Asterisk source?

Your comments are appreciated.

Kind Regards,
Hohenzolern