Port forwarding question fortigate 80C

Hi all,

I have pbx that is connected to a network with a fortgate 80C.

The lan side of pbx is 172.50.60.5 and the WAN side can be for example 182.83.45.99.

I have configured on a laptop that X-LITE soft client can register on the pbx through the wireless network and also when connected to the network physically.

It can make a receive calls great as long as your on the network.

What I want know is how can I set up a port forwarding in the firewall, so that I can use X-lite anywhere there is internet to make calls.

How can I configure this and are there other things that I need?

Thank in advance