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I have a similar problem, but on calls between two extensions. I have installed Axon Virtual PBX on my system running Windows XP Professional, SP3 with .NET 4.0. I have registered two Androiod smartphone as extensions, by installing CSipSimple on the phones. I am able to make calls to and from the extensions, but the calls only last for 6-8 secs, while the PBX panel shows the following message, 'Destination of message from user (extension number)@(IP address) cannot be determined'. The extensions also show as offline in the System Status. I have tried disconnecting and re-registering the extensions and restarting the Axon PBX software, but to no avail. How can this problem be rectified?