[Asterisk-Users] dialing sip before answering pstn line

[bman at host.sk]

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....