If your iPhone won’t get service even though it is under contract, there are a few things you can do to fix that issue. To begin with, restart your iPhone and see if service is restored when it is started back up. If that doesn’t work, remove the SIM card and make sure it is a valid card and that it is not damaged, worn or modified in any way. If it looks fine, put it back in and check for any carrier settings updates.
You can check for carrier settings updates by tapping Settings, General and then About. This needs to be done while you are connected to Wi-Fi or through connecting to iTunes. If any updates are available, you will see a message and can prompt your iPhone to update.
If updating the carrier settings still won’t help you get service for your iPhone, update to the latest iOS version and finally, restore your iPhone if nothing else has worked. If at that point if you are still having No Service issues, contact your carrier to see if there are any problems with your account or with the network.
Source Apple Support