Sorry maybe I was not clear, I will explain further:
Case A
If you receive an audio call invite (ie. from a deskphone), of course you will only see the "answer" button appearing on the desktop client. If you press the Sennheiser headset button it will answer the call.
Case B
If you receive a video call invite (ie. from the webclient), the desktop client shows 2 options so you can choose to either answer in audio or video mode. If you press the Sennheiser headset button it will answer the call as an audio call by default.
This is tested and it behaves this way that I described, on the latest software as seen below:
Headset Sennheiser DW Pro 2 with FW v160
Windows client 16.0.1.81
HeadSetup Pro 3.2(11230)
Sennheiser 3CX Plugin 3.2.9301
PBX V16.0.2.910
When I said unrelated I meant that whether you are offered a call as only audio or as audio+video there is no special default for case A or case B that would make a difference. If you press the button on the headset, you will pick up the call in audio mode for both cases always because it's effectively the same thing. I hope this was more concise, let me know if you need any further clarification