|Plantronics and Avaya are two separate companies. My guess is that you bought a Plantronics cordless headset that works on Avaya phones.|
That would explain the smaller connector. The small connector plugs into where the handheld handset plugs into on the telephone, not into the telephone line. I didn't find your model number on Avaya's or Plantronics site but you need to determine what you really bought. I don't see anywhere on their website that they sell a regular cordless phone.