If you are not using limiters, then note this from the guide;

The ALTQ framework is handled through pf and is closely tied to network card drivers. ALTQ can handle several types of schedulers and queue layouts. The traffic shaper wizard configures ALTQ and gives firewall administrators the ability to quickly configure QoS for common scenarios, and it allows custom rules for more complex tasks. ALTQ is inefficient, however, so the maximum potential throughput of a firewall is lowered significantly when it is active.

pfSense software also supports a separate shaper concept called Limiters. Limiters enforce hard bandwidth limits for a group or on a per-IP address or network basis. Inside of those bandwidth limits, limiters can also manage traffic priorities.

Hey @Derelict Thanks for the ideas. I've managed to find the issue, and it was an equipment problem on our ISP side.

I ran some packet captures on both the internal VoIP VLAN, and the external SBC interface(our phone comes in over the same fiber asour IP, but on a separate interface on the demarcation unit(T-340)), found a lot of dropped packets, moved onto our WAN switch, and saw that there was an EXCESSIVE amount of Tx/Rx errors and collisions on the demarcation interface. When I dug into the interface itself, found that it was only negotiating at half-duplex, which would explain the issues. When I went back through my old config files for the SG-8860, I found that I had to force it into full-duplex mode as the T-340 for some reason will not auto-negotiate to full-duplex, but if I force my side to full, it works just fine, and not a single error since.

Thanks again for chiming in.