Sometimes, you will get an error that states the Apple activation server is unavailable. There have been times in the past when a new iPhone was released and Apple's activation servers were struggling to keep up with demand. In this situation, users would have to wait for their activation. Another common issue with this error is actually due to hardware failure. Assuming you've tried all of the steps above, you may want to contact Apple. Although your carrier may have to replace the phone (if it's within the return period), Apple should be able to provide some clarity on this matter.