Incoming audio and calls

Hi everyone, i cant seem to make incoming calls or audio work.

I have an asterixNow installation behind a firewall using NAT - with NAT rules sending all packets on ports 5600-6000 and 8000-20000 to the system.

I am able to call out from internal clients, and can hear my voice on the recieving PSTN handset, but not audio is being returned. Im theres two issues are related.

Does anyone know of a step by step guide for setting up incoming SIP, i have 2 books on asterisk and clearly state the main causes of problems like this

Help much appreciated