You can exit out of the subscriber conversation to the opening greeting... if you hit * once from the "top level" subscriber menu you go to the opening greeting - if you hit * again, you go to the sign in... and if you hit * again, then it hangs up. Sounds like folks may be whacking one too many *s there.
Just as an aside: In 4.0(1) you can define the exit destination of the subscriber conversation to be something other than the opening greeting - as such you'd be able to send it to a call handler that didn't have * defined as subscriber sign in which would avoid problems with too many *s being hit.