SIP and hangup problems with POTS phones


I have now asterisk 1.6 (some problem with 1.4) and i’m using AGI scripts to implement a more sofisticated callback system.

This system only use SIP, to make and receive the calls

And the problem is:

If I receive a call from asterisk in a POTS phone, and answer it, I can hear the welcome message, and the other messages saying that the system will connect to the other party, costs, etc.
if during this time, I hangup the POTS phone, the agi script continues till the end, and try to establish the connection to the other partie !!!

If I receive the call im a mobile phone, and press the hangup key, the agi script ends right away!

So my question is, what is wrong, for the hook button in the POTS phone, to not hangup the call


PS: I have no dadhi (zaptel) configuration, because I believe, using only SIP, to receive and make calls from and to the outside, it is no necessary