It is definitely possible to use USB ports while having ThunderBolt to Eth in.
The problem you may be having is the conference phone which would probably appear as a communication devices in your Networks.
What you can try is: connect both cables TB and USB
in System Preferences > Networks
Set Service Order (cog icon bottom of the window)
This changes the preferable connection device.
See if that helps - otherwise this is some kind of conflict with third party conference phone.
Post back your results.