Could Not Connect Siri to the Network

My sister and I are both using iPhone 4S under the same network and plan. The problem is her Siri could not connect to the network. Mine is working correctly. As I am trying to help her with her problem, we have done countless reboots and resets. It still says, cannot connect and we should try later. How can we fix it? I heard a lot of complains about Siri having issue with the server, I am just wondering if Apple is aware of that. Do you have any suggestions?
