I've never seen this happen on emulator but on device it's quite easy to test. If you open a connection to a specific IAP and then send a HTTP Post using that connection. Then read the response in a normal way. After that send a SMS to your device. It should cause KDataTransferTemporaryBlocked event to occur when monitoring the line status.
Now, if you make a second HTTP Post on that open connection you should get a KErrDisconnected event in your MHFRunL() method. The strange thing is that this happens from time to time, so it's not really a consistent feature.