I'm assuming they gave you a softphone and not hardphone for a reason. So a true handset probably won't work...
The good news is that you can get just about any usb handset and use it for the ear and mic, although the buttons may not work with the softphone client.

What I do with softphone is use a bluetooth headset. Just pair a BT headset to the computer. You may have to tell the softphone to use the headset too, but that should be trivial.