PBX config for VOIP Gateway

I need help with configuring Intertel Axxent with an external VOIP gateway. I have the gateway configured, and have already set up the FXS ports as CO lines as a trunk group on both ends. But I’m confused about how to interconnect the PBX w/ the FXO ports on the gateway. I think this will give me added functionality. Do I connect to an PBX extension port at each end, or do I have to connect one end to a PBX phone and the other end to the PBX? Is the extension port the right interface for FXO? Any help steering me in the right direction will be appreciated.
Thanks!