asterisk users - Sep 2005 - dialing sip before answering pstn line

Hello,

 I have asterisk server with two isdn bri cards (billion) using zaphfc 
driver. Also I have from telephone
 company routed (for example) 16 pstn numbers. It is technically 
possible to dial SIP phone from outside
 before answering isdn pstn line.

I have local numbers 201,202,203 and from telecomunication company 
numbers 555201,555202,555203.

I don't want to be "pstn" caller billed if SIP user don't
answer.

It is possible? thanks....