How to fix echo on calls using headset with VVX 400?

I have 1 VVX 400 for my use at home, and when I use a headset, the person I'm speaking to hears an echo of themselves.

I've already turned on "Echo Cancellation" with no luck.

It sounds similar to this issue.

• Software Version: 5.9.2.3446
• Model: VVX 400
• Call Platform: Skype for Business.
• No additional infrastructure
• Backup attached. Domain name removed for privacy