I am running AstLinux on another box behind Zeroshell just fine.
In the “pre boot” script I have:
To make the SIP data get through NAT properly. No idea why that is not standard in ZS.
Under “Virtual Servers” I have UDP port 5060 forwarded to my Asterisk box. You may not need that unless you are registering phones to your Asterisk box from outside or are allowing unsolicted calls from others (maybe via e164.org, etc.).
That is about it. Oh, yes. On Asterisk, I don’t bother telling it that it is behind NAT as the nf_nat_sip on ZS takes care of spoofing that.