We have an article that goes over troubleshooting iPhone activation issues here:
Perform the following steps:
- Restart the iPhone.
- Try another means of reaching the activation server and attempt to activate.
- Try connecting to Wi-Fi if you're unable to activate using a cellular data connection.
- Try connecting to iTunes if you're unable to activate using Wi-Fi.
- Restore the iPhone.
If you receive an alert message when you attempt to activate your iPhone, try to place the iPhone in recovery mode and perform a restore. If you're still unable to complete the setup assistant due to an activation error, contact Apple for assistance.
Hope that helps,