My headset won't work in my cordless phone (it used to)?

I have a PhoneMate cordless phone with answering machine. Model No. PM5851. 5.8GHZ. It was brand new on Christmas 2008, so only a year old. I use a headset plugged into the side. About 4 monthes ago, the headset started making a very loud ringing sound. It is heard by the other party as well. I bought a new headset, and am still having the same problem. Can anyone help?