I've been on quic for a few months now, and generally very happy with it.

I'm on a 300/100 plan, using Ethernet to an RT-AC87U that usually reliably handles the bandwidth.

However, in the last couple of days, VOIP quality to Japan (using all of Signal, Viber, Skype, Google Meet, Jitsi) has been uniformly awful downstream only. My business partner can see and hear me fine, but I can't see or hear him with any reliability. Audio is choppy, video looks like 240p resolution. I'm trying to use this for business meetings, which is a problem.

Other points: High-end computers on both ends (less than a year old, everything up to date) with 2.5 gig ethernet connection.
I've swapped out the router for a known good Asus router, no change.
Using my laptop with Wi-Fi and ethernet, no change.
Speedtests to various providers show full 300/100 speeds, and they are as they should be.
I tried calling a mate in Sydney on Signal and FB messenger. Same result, no change.

My business partner has tried various devices at his end (laptop, desktop, phone, on ethernet, Wi-Fi, and data) with no improvement.

I've tethered both my desktop and laptop through my phone connection, and it works perfectly, eliminating the router(s) and the provider as the problem.
The process of elimination suggests that the only issue left is with my provider.
Am I missing something?