Let me start off by saying I am not an iPhone user.
I just set up my mom's new iPhone 4S and during the process, I encountered some difficulty. I spoke with customer service and he gave me some tips that were very useful.
1) An internet connection is MANDATORY to set up this phone.
2) Turn the phone off and then back on after completion of the setup process.
My main problem was that I could make outgoing calls, but not receive calls, on her phone. I followed the advice in this post:
and basically started over from scratch. After completion, I restarted the phone (just like your computer after an update) and it worked perfectly.
I know this will not help everyone. But it is somewhere to start. Good Luck!