I have had enormous success with the Tell Bell! I created a thread on it just the other day to mention it to others
http://www.yorkietalk.com/forums/tra...tell-bell.html
I'll also add that I've traveled with Cooper's bell. I brought it to my mother's house and put it at her front door. He rang it to go potty. When i took a trip to Europe a few weeks ago I brought the Tell Bell to Cooper's kennel. I explained to them what it was. They said he rang it all day long, just so he could get out of his cage!