Polycom SIP Phone to AskoziaPBX over pfSense VPN

I'm surprised this doesn't "just work", but so far that's what I'm experiencing. I can dial out, but I get no audio back, a.k.a. one-way audio.

Here are a few links I found which allude to the problem I'm experiencing:

Asterisk and IPCop VPN one way audio problem

One-way audio with VPN

Multiple Locations (VPN or multiple trixbox servers?)