Vista - Getting OperationContext.Current on connection

Hello
In all the samples of duplex the callback is set using
OperationContext.Current inside a method called by the remote application.
However, this approach requires the client (initiator) to initiate a call to
the server before the server can use the callback to send messages to the
client.
How can I enabled the server to send messages to the client using the
callback after the client connects, but BEFORE the client sends anything?