I connect the Plantronics Voyager UC Headset to my PC through the BT600 dongle. I can hear fine call audio, but my voice is not transmitted to the person I am calling. Same for inbound calls.
Here is the message that I get when I try to place an outbound call with the RC Desktop PC App:
Plantronics Hub: I have uninstalled it, re-booted my PC, re-installed it, and confirmed the version is current (V.1509).
Voyager UC: Firmware: V.236 - current. Battery is fully charged. Headset is within 4 feet of BT600 dongle. "Sensing" is turned off - I answer calls manually, not by putting on the headset.
Ring Central App version: 10.2.1.31914 - current.
Ring Central Microphone Settings: Selecting either one of the two available settings [the "same as system" setting (which is Plantronics BT600), or the "Headset Microphone (Plantronics BT600)"] does not make any difference.
PC & O/S: Windows 10 Professional 64 bit running on Dell M2800 Precision Mobile Workstation. Windows has all updates except Cumulative Update for Windows 10 Version 1803 for x64-based Systems because Dell has not done the drivers for that.
Windows Audio: The BT600 is set as the default microphone
The laptop has Bluetooth, but it is always disabled, so that does not play a role in this issue.
When I connect the Voyager UC to my cell phone via Bluetooth, the microphone works for both RingCentral Android App calls and for plain cellular calls. So, I am confident that the Voyager UC is not physically broken.