[Asterisk-Users] DTMF information?

[Chris Lee]

I am looking at building an IVR product with a few interesting features 
and need some more information about how asterisk and VoIP work and what 
I can get from them.

As far as I can tell when I use ISDN/GSM telephone networks the DTMF 
information travels as data representing 'start tone' and 'stop tone' 
for each button pressed, it is then generated at the other end if an 
audio representation is required.
I am interested to know if I can get access to these events 'start tone' 
and 'stop tone' through the dialplan or an AGI or by acting as a VoIP 
device. Or of course if I am completely off track and should give up now.

I am looking to get the length of time a button was held down rather 
than that it was pressed.

Thanks for any help

Chris.