My Work Phone's calls are being picked up by an automated answer machine?

I have an Avaya Partner 18D phone here at work, I had reset the phones accidentally. (i'm the new receptionist.) Some calls, NOT ALL, when they come in, the phone rings once and it is picked up for some reason by i dont know what! I tried calling from my cell, and it's an automated message machine that says "party is unavailable, press another parties extension" but we don't have assigned extensions. I need it off!!