I think the OP may have been inquiring about the Electronic Hookswitch feature, which is usually off by default and must be enabled in the Phone Configuration screen by the CUCM Administrator.
This feature also requires a special EHS adaptor cable from the headset manufacturer, Plantronics p/n 38350-01 for the Plantronics Savi series headsets. There is a different cable required for the CS55 series headsets.
Original MUG/NAMU Charter Member