Both of these need to be installed to enable call control features from the 3CX Windows client. These don’t appear to be required to use the web client or the Chrome browser extension.
Sennheiser Headsetup Pro
https://www.eposaudio.com/en/nz/enterprise/technology/software
Sennheiser 3CX plugin
https://en-nz.sennheiser.com/software-3cxphone