WCF Callback Functions

I am working though Lab 6658 for my 70-569 exam (due in January).  I am having a problem getting the Callback functions to work.  Granted there are more moving parts using a callback function that a request/response pattern, but I think I have isolated the problem.  The start kit that Microsoft provides does not use a Duplex Chanel – which I thought was necessary for callback functions.  I will investigate on my local machine.