VoIP RTP feedback/echo – RTP voice stream from remote caller is amplified and reflected back to them

        I have a specific issue where calls start to deteriorate after about 20 seconds, then usually get progressively worse. I'm looking for help in narrowing down the cause. The deployment is a cloud PBX, and the phone on site is a Yealink T41S.
Here is a PCAP from the cloud PBX, showing both streams. Blue is the stream going to the T41S. Grey stream is coming back from their network to the PBX. PCAP from Cloud PBX showing RTP stream The T41S user hears choppy sound, and the remote caller can sometimes hear awful, amplified echo of their own voice. Once I had to take my headset off just to speak to the user. As you can see, when the T41S user is talking, it's fine, but when the remote party speaks, the signal comes right back at them, amplified. I've tried using no handset, with handset, using another brand new T41S phone, but with the same result each time - which is why I suspect a network infrastructure issue. These calls are routing through 4 large network switches on site, then a fibre-link to another switch, then to the VDSL modem. I've got no idea how the LAN is configured, but the phone isn't VLAN tagged. I'm looking for suggestions for what could be implicated here, and what causes this type of behavior. Please share any insights, it would be a big help. Cheers

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