Note: As with (almost) all Cisco hookswitches, in order for this to work, you have to have hookswitch mode enabled in the CUCM (Cisco Unified Communications Manager), which can usually only be accessed by IT. Luckily, if one phone in the office already has a hookswitch on the Cisco phone, then the setting has likely already been put in place.
Installing the EHS:
1. Plug large end into the Plantronics base (will look like 2 cords fused together)
2. On the other end of the cord, plug the RJ9 end into the headset port of phone
3. Plug remaining end in "EXT" port on Cisco phone (may also say "AUX" or "EHS")
4. Make sure the compat on base is set to either E or F (Usually E)
Assuming the first step (at the very top of the page) was taken by your IT, this should get it working right away. If it doesn't open the line when headset is turned on, press headset button on the phone to confirm that the rest of the headset is working.
In case you are upset about this fact, here is a picture of a puppy.