At the time, I also bought the Poly adapter to connect to my phone. Here is the description of the Poly adapter: The Plantronics adapter cable is suitable for various Plantronics headsets. The cable has a QD plug (Quick Disconnect) and a 2.5 mm plug for connection to DECT telephones, e.g. from Panasonic.
Manufacturer Poly
Category Headset accessories
Manufacturer no. 64279-02
Release date 02/06/2014
Also ordered from digitec.
Everything works well, although I had to send the Poly adapter back once because it was defective after a short time. However, I have been using the replacement for some time now and everything works perfectly.

Sorry, no idea.