Partner headset adapters plug into the aux port on the base of system telephones. On the adapter is an "on/off" button that picks up the call without lifting the handset.
This is good and bad, in that some folks just have to have a release key on the set so they can do all phone functions from the set without having to touch the adapter as well, but it also allows the use of just about any single line device in the place of expensive headset only bases.
Many of my customers rely on clip-on-belt style GE cordless phones with headsets at ~$65 for the whole setup. Not only a headset, but cordless as well! Try that with a Plantronics and you'll pay better than $200. But, I sell a lot of Supras and Mirages, too with the on/off base, so to each their own.