Since Downloading IOS7 On iPhone 4S my FaceTime does not work! Now FaceTime actually have the icon on the home page it doesn't work! I click on the icon and it tells me sign in, so I enter my password and after about 30 seconds it says check network setti

Since I downloaded IOS7 on my iPhone 4S my FaceTime does not work.. Now there is an icon on the homepage I can't seem to make it work. I click on the icon and it tells me to sign in, so I type my password and after about 30 seconds a pop up box comes up and says " FaceTime activation ... Could not sign in please check network connection and try again" I know my Apple ID and password is correct as I can still download apps and music it's just this what does not work.. I downloaded IOS7 on my iPad last night and that worked straight away I didn't have to sign in or anything. Please someone help me...