invoke callback during the execution of a contract operation
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I am programming a WCF service on Azure. 

In my Service, I need to invoke callback during the execution of a contract operation. When I attempt to do that, an exception will throw and the client will locked. I think it is caused by that the channel is opened for contract operation, invoking callback in current channel will lock the thread, am I right? I want to get solution for this scenario.
 
here is the timeout exception message:
 
This request operation sent to net.tcp://127.255.0.0:8000/MytestWCFService did not receive a reply within the configured timeout (00:00:59.9889989).  The time allotted to this operation may have been a portion of a longer timeout.  This may be because the service is still processing the operation or because the service was unable to send a reply message.  Please consider increasing the operation timeout (by casting the channel/proxy to IContextChannel and setting the OperationTimeout property) and ensure that the service is able to connect to the client.
