Good afternoon.
Series phones have SPA50x standard connector 2.5 stereo headset. I have a headset Jabra PRO 920 and huchu connected through the standard connector to the phone.
But for this you need an adapter Jack 2,5 -> RJ12. One such adapter I made myself a selection method which contacts with which now connects. And now you want to connect another 10 headsets. Maybe there is some std.
