I had something sort of similar if I have got your need right.
It was what we called a Madonna headset.
It clips onto the user port on the DEC phone which then stays clipped to your belt unless you want to dial a number that isn't a voice activated speed number.
Spent nearly a year and half with this set up. Left the dec on auto answer 5 rings. Leaves both hands free for keyboards and also doesn't mean you get a knackard back and shoulders from clamping the phone to your ear.
Only thing to watch though is you forget to turn it off auto answer when you go to the bog.
Just did a quick search and found this site
Phone Headsets - Nicomm
Or if you want a cordless extension
Wireless Telephone Extensions
I use one of these so I can get telephone control over my central heating system. Means then you can plug a normal fixed line phone in and run a headset of it.