Hi Steve,
Pexip doesn't support NAT for H323 calls, which means that for an H323 call to work from a video system on a private (NAT'd) address, you will have to configure NAT on the Polycom device itself, and then configure appropriate port forwards on pfSense, so that media and signalling sent from Pexip will reach your Polycom device (I'm assuming here that your Group 310 is sitting somewhere on your internal network).
Let me know if that makes sense!
Edit: As you mention, SIP should also work fine since Pexip does support NAT/FW traversal for SIP calls (Is there a reason you prefer using H323 rather than SIP here?)
-Andreas