To protect your SIM card from others using it for phone calls or cellular data, you can use a SIM PIN. Then, every time you restart your device or remove the SIM card, your SIM card will automatically lock and you’ll see Locked SIM in the status bar.
How to Turn your SIM PIN ON or OFF on iPhone or iPad
In iPhone, go to Settings >> Phone >> SIM PIN.
In iPad, go to Settings > Cellular Data > SIM PIN.