Several things can cause this. None of them are glitches, but just the way it is supposed to work. If a phone is programmed to ring for an incoming line, and that phone is also programmed to "foward no answer" to voice mail the incoming line will be anwered by the extensions personal greeting when the foward delay is reached. There are several things you can do to avoid this scenario.
1. set the extension foward delay to more than the AA delay, or more rings than the operater needs to answer the calls.
2. Remove ringing line appearencs from the set.
3. Also make sure of your DRT to prime settings, if you are using DRT to prime.
number one will most likely work for you.