hi,
i believe, on PIX you are NATing the internal polycom to some public/other IP. and the ACL rules and NAT statements are not applied as per that
Incase, you want the foreign polycom to access internal poly with its original IP, then you need to setup "nat 0" or identity NAT .
It might be possible that in netscreen, these conditions are already met and hence working. kindly check this.
regards
aashish C