I just wanted to add to this thread, since it's what shows up at the top of a google search for PFSense and VOIPo. I have a GrandStream TA.

I was experiencing 1 way audio on inbound calls once I installed PFSense. I noticed looking at the firewall logs that the external servers were attempting to connect on low ports outside the "standard" port forward ranges VOIPo recommends. The problem is caused by an outbound port remapping feature, which is enabled by default for security reasons:

https://doc.pfsense.org/index.php/Static_Port

This link describes the feature and includes instructions on enabling a "static port" which disables this feature for a specific device. Once you do that, the generally prescribed port forwarding options will work and the 1 way audio problems stop.

I had seen a few other posts suggesting setting up some specific mappings for specific external servers, but I didn't want a solution that was dependent on knowing external IPs. VOIPo might change those. I figured it had to be something simpler, since the port forwards work OK with most generic routers