What type of app are you writing? Windows Phone Silverlight or Windows Phone Runtime?
Since you posted in the Windows Store apps forum I'll guess Windows Phone Runtime. To intercept frames you'll need to write a Media Foundation Transform (MFT) in C++ and register it as a media extension to call from the MediaCapture element.
There is a Media extensions sample which demonstrates how to do this on Windows. The extension code itself will be different on the phone, although the framework
for the sample will need to be rewritten.