As well as port 6000, you also need to map ports 30,000 - 30,031 through tho the OAS card IP address (If separate card), if it is a 7030, 7100 or 7200s, and you are using the inbuild MGI ports, them NAT them through to the system IP address. Each MGI/OAS port on the card requires 2 channels, hence the 30,000 - 30,031 for a 16 port card.