If I'm on my desktop I would prefer to use my Desktop with it's headset for calls. I mostly use my Laptop together with my phone. Beside normal use of KDEconnect over W-Lan/LAN also connect over bluetooth and fake a headset profil or do it over USB (Audiorouting), so I could use my desktops headset to hear and desktop SIP-Software to stear my phone. -> With audio routing and KDEconnect you have all you need. Put them together and Fake a lokalhost SIP-Server where you could connect normal SIP Software Add a SIP-Server module to KDEConnect Allow inside this modul to assign either a Bluetooth or an USB Device to a connected KDE-Connect device If a KDEconnect device appear check if it also could get accessed by Bluetooth or USB and if successfully start the fake SIP server.
I'm guessing PipeWire can help with that.