I tested just now in Teams version 1.3.00.362 (64 bit) and was able to answer an incoming call just by lifting the handset. This is different to about two weeks ago when the hookswitch was completely ignored. This works even when the CX300 is not my default audio device in Teams.
Hanging up the handset does not end the call, it instead transfers it to the CX300's speaker. The mute button does now work. I also tested with the Plantronics 540-M and got the same results.
So, there's some progress! Someone at Microsoft is working on this. Hold on to your hope people