Couldn't activate iPhone . activation server cannot be reached.

my iPhone 4s shows me .... update completed. your iPhone was successfully. There are just a few more steps to follow, and then you're done !

Continue......

then I connect with my WiFi and Next But its Shows me Your iPhone could not be activated because the activation server cannot be reached. Try connecting your iPhone to iTunes to activate it, or try again in a couple of minutes.

If this problem persists, contact apple support at apple.com/support

tell me what can I do....