Plantronics Bluetooth headsets stopped working after my last ios update

I've used Plantronics 5200 Bluetooth headsets with my iPhones for years without problems on AT&T's network. The headset worked fine on my current iPhone X, which has never been unlocked or otherwise hacked... until the latest Apple update to ios 15.3

Now, I can hear people perfectly, but they can’t hear me. Siri voice commands and other voice inputs aren't being heard, either.

I've updated the headsets' software, reset its sensors and used Plantronics "Hub" to test its operation. Everything is A-okay with the unit.

Help!

Thanks!