asterisk users - Apr 2006 - Is there a way to monitor the DTMF tones on a channel?

Is there a way to monitor the DTMF tones on a channel?

I have a prepaid application working in asterisk. When the user dials a call and
wants to cancel the call before it is answered, there is now way to do it
without hanging up and redialling the access number.

Is there way to monitor a sequence of DTMF tones and cancel the call?

If I use a SIP gateway or proxy rather than dial asterisk directly will that be
possible?