I absolutely love the 4025, I have two. I use the green pair for the 2nd one to call people back while on hold with service providers. I have a 4150 that I thought I would like and went back. I was curious because I have two 8520's in the back office. The other sites have axxess cabinets. I wanted to test them before sending them off-site to one that could use them. There's a sound of plastic pieces inside.
My boss isn't thrilled with the quality of conference calls lately. He has a 4025 too. We tried a mitel 5330 ip set, but he got confused.