Try Plantronics website maybe.

In om,
an infinite number of Meowws, in an attempt to produce the entire works of
Shakespeare, typed:
I would like to replace some of the lift up handsets on our office
telephones with headsets. Has anyone ever done this, is it easy and
where can you get them from?

Our phone system is a rather old Philips 'Sopho' system with four
extensions on it and three incoming lines and I'm not sure whether it
would work. For instance, how do you 'Pick-up' and 'hang-up' a call
without the handset. Or is it a case of using the handset and headset
in parallel?

Any ideas?

Patrick