I have had a few questions from Game developers asking about the use of networking classes in System.Net
Some of the name spaces they are trying to utilise are simply not available on in .Net for Windows Phone or they have to changed from Windows 8 to WinRT. The following Unity project sample below includes complete implementations for System.Net.TCPClient using Windows.Networking.Sockets WinRT namespace: /UnityPorting/tree/master/PlatformerPlugin/MyPluginUnity/Legacy/System/Net WinRT has new sockets APIs in Windows.Networking namespace and you should be able to extend on the approach demonstrated in the sample project functionality.
Unity Windows Phone Game Sample https://github.com/windowsgamessamples
If your porting from Windows 8 to Windows Phone then each platform utilise different name spaces so you will need to find alternatives using windows. namespaces from WinRT.
Here is a nice table of mappings http://msdn.microsoft.com/en-us/library/windows/apps/br230302.aspx