Go to Settings, turn on the Airplane mode for a few seconds, and then, turn it off. It is simple as that.
Reboot your iPhone.
Sometimes, the SIM card may move a bit. Open your phone, remove the SIM card, and place it back again properly.
In many cases, when you update the carrier settings, the issue is gone. To do this:
- Go to Settings
- Tap on General
- Tap on About
First, check if you have the latest version of iTunes, and back up your data just in case. Connect the phone to your computer via USB cable. Once it is connected, choose your device in iTunes. Find Restore in the Summary pane, and click on it. Approve it. Your device will restart. Set up your device, and you are done.