This worked for me on a iPhone 5 I purchased on eBay, thought it might help someone else as well...
http://www.iphonehacks.com/2013/10/fix-activation-error-ios-7.html
"How to Fix Activation Problems with iOS7"
1. Download the appropriate, and very large file
2. Backup your iPhone
3. Hold Shift key, click on Restore iPhone
4. Select file you downloaded, and viola...
5. Restore your phone using your most recent backup
http://www.iphonehacks.com/2013/10/fix-activation-error-ios-7.html
"How to Fix Activation Problems with iOS7"
1. Download the appropriate, and very large file
2. Backup your iPhone
3. Hold Shift key, click on Restore iPhone
4. Select file you downloaded, and viola...
5. Restore your phone using your most recent backup
