Here is the list of points to consider before building Callback Contract type of Service or Publish and Subscriber model.
Callbacks requires communication over duplex channel .
Requires all clients to implement the callback interface, exposed by the service.
BasicHttpBinding & WSHttpBinding don't support callbacks -- use their Dual versions instead.
Streaming data not supported by callbacks . streaming is not reliable and Callbacks require reliable messaging.
If the service restarts, callbacks are not possible until the client makes a service connection again & subscribes itself for callbacks.