Initiate 3-way call, and hang up: how to disconnect everyone ?

[kim8888]

hello

we are using asterisk 1.6 pbx and polycom soundpoint ip550 phones.
asterisk is connected to a full PRI.
the outbound dialplan is very straight forward: _NXXNXXXXXX,1,Dial(DAHDI/g1/1${EXTEN)}
it allows us to place outbound calls no problem
i can also do 3-way call like this:
* call a first person
* press conference on the polycom phone
* call a second person
* press conference again on the polycom phone
* and now the 2 person plus me is on a 3-way call

here is the problem:
* if i hangup the phone while all 3 participants are in the call, my line will be disconnected from asterisk pbx, but the other 2 outside person will still be on the call... using the minutes of our phone bill, because we did two outbound phone calls.

how do we configure asterisk or the polycom phones to disconnect everyone when the person who originated the call from a polycom phone hangs up ? we don't want to have to do this manually by splitting the callees, manually hanging up to each of them, and then hanging up our phones....

thanks to all