I have an OKI 900. In it's day it was pretty fun, you could unlock the ability to scan the cell towers with it so you could spend time walking around listening to other people's calls. Power it up. Then hit 7 and 9 together. MNU, SND, END, RCL, STO, CLR. Then 1 and 3 at the same time. The screen says "good timing!!!!" and you have access to all kinds of neat things, I think you could even change your own phone number. The batteries I have don't hold a charge anymore so I haven't fussed with it in years.

Aside from my OKI I have never used one and don't intend to.