So I do see useinbandfec advertised in the SIP INVITE from for both freepbx and Voiper, but it doesn't sound like its using any at all.
I use Network Link Conditioner for mac os, join its wifi sharing and add 10% packet loss both up and down. packet_loss=30 is set in codecs.conf for freepbx, the quality should still be very good based on the OPUS comparison page they have.
Quality is not good. Is voiper's opus compiled with packet_loss >= 10 as well?