iPhone 4 activation error

Hello,

I've iphone 4 bought it from customer. When I checked everything was working alright. I wanted to erase everything from the phone so I went to settings/general/reset and tried to reset the phone but it asked me to put passcode which it shouldn't, right? Then I tried to hard reset connected to ITunes. After sometime while restoring it gives me the following message

We're sorry, we are unable to continue with your activation at this time.
Please try again later, or contact customer care.

Then I unplugged the usb port from laptop and restarts and ask me to activate the phone . I followed the steps
English
United States
Choose a Network ( I choose my Wi_Fi)

It ran for few seconds and then the following message pops up

Activation Error

Activation request could not be completed. If the problem persists, please contact customer service.

I'm very confused and frustrated as I tried it so many times. Could you please advice me what seems to be the problem and how to resolve it. I'll really really appreciate your help on this.

Thanks,

Sid