If you get your Grandstream registered, you should be able to make inbound/outbound calls. You would just need to create the extensions and make sure the trunk connected to an outbound route that has a routable dial pattern. This can be done in the GUI.
A quick Google search revealed:
https://wiki.freepbx.org/display/FOP/Configuring+a+Grandstream+GXW-410X+Device+to+act+as+an+FXO+Gateway
These cover registering each jack as a SIP account or setting up the Grandstream as a PEER gateway.