There are no USB handsets supported by 3CX at this time. Maybe others have some experience using them and can give you an idea of what does or does not work. Just keep in mind that 3CX is an IP based phone system using SIP protocols.
We have made Plantronics Blackwire 310 USB headsets work by using the old (unsupported) Spokes software. Also the CS-50USB wireless headsets work with this software too, but they are a little flakier.
Don't know about the headsets working. You might contact Plantronics (maybe a firmware option) for advice or get one from the client and test accordingly.
We are running Lync with the most current configuration through Office 365 on E4 educational accounts. We can use Lync through USB specific phones built to connect to Lync via a USB connected computer.
We just purchased some Ubiquiti UVP android phones capable of running Lync via android. I am assuming that those phones will continue to work with Lync while connected to a computer regardless of 3CX.
Just my 2 cents thinking out of the box on a unorthodox and mousetrap setup.