How can I call be terminated during the ringing phase?

How can a call be terminated during the ringing phase, before it is answered?

Is there a way to monitor the dtmf tones during this phase?

What is the purpose of killing a call during the ringing stage = under what circumstances?

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

If the user wants to make a number of calls after dialling the access number and a call is unanswered after some time, or realises that he dialled the wrong number, the user can cancel that call and make another call, without having to hang up, dial the access number again, and reenter the PIN number.