I have a northern telecom key system which has 3 jacks on the phone - one for the line, one for the handset, and a 3rd for the HEADSET (same size jack as the handset).(UPDATE 11/2009) - having used this to hook my normally very loud headset to my northern telcom phone system I finally went out and bought the amplifierPlantronics M22 Amplifier- there were 2 reasons for this. Although I could adjust my hearing volume on the phone set, people could not hear me as well on the headset as with the phone's normal handset, which is backwards of the way it should be. The amplifier has an adjustment for the microphone gain which affects how loud you are to the person on the other end. Second, by switching off the headset at the amplifier I regained the use of the phone's built in excellent quality speakerphone. Speakerphones that sound good are rare and I particularly liked this one so now I can choose between headset or speakerphone when I press the handsfree button on the key phone.The guides say use a plantronics A10 and a plantronics H-series headset to plug in the hole, but nothing more. Whats worse is that since it works with multiple keysystems, plantronics can't predict how they will work together, so this review is a NT/Nortel and A10 and H series combo pack review.Functionally the A10 makes a H series headset work like a personal speakerphone - it defeats the speakerphone in the phone itself, and functions that get disabled when using the speakerphone also get disabled using the headset which is why I say it acts like a speakerphone.Also your phone's speakerphone quits working when the headset is plugged inAnd finally you must turn on auto handsfree as a feature on the phone for the headset to work (the LCD display will tell you this as soon as you plug it in)Otherwise, it saves you the expense of buying a 100 dollarish plantronics amplifier and flip the switch all the time, deal with batteries or wall warts, etc. If you are on the phone all the day, its a great timesaver, but if you like your speakerphone, avoid it and go the amplifier way. You can still use the speakerphone that way!My email is in my profile - feel free to write if you are also using nortel / NT equipment and have any questions about the headset. It sounds great!