Phone question - Avaya brand headset prevents me from using hand held phone?

At work I have an Avaya phone and head set. While for long calls I like the headphone option, I would like to be able to answer calls by just picking up the hand held phone. I can hear a dial tone and what people are saying on the other line, but they cannot hear what I am saying. The headset didn't come with instructions on how to handle this. Does anyone know how to let me use my hand held phone???