Some of the DECT headsets I've seen, are designed to plug into the handset socket of a desk phone, so a model that plugs into the headset and Mic connectors of a PC, is going to narrow down the short-list. Add to that a requirement that it must allow the headset to answer the call, well, that's going to be tough, as there would have to be more than just the audio connections to accomplish that. Unlike Bluetooth, there is no "default" data connection to signal on/off-hook.
You might want to mention in which part of the world you are located as the frequencies for DECT worldwide, vary by region. A product available in one country, may not be available in another.