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.