I am using ZS since 1b14 with Linksys PAP2T ATA (SIP adapter) and in all these years I never needed to set up NAT for the ATA. It simply works fine without any settings on ZS side, don’t know why.
Also, there is a lot of P2P traffic on the net and the machine ZS runs on is very low on CPU utilization (it’s P4 2.2 GHz with 512 MB RAM).