Short for embedded SIM, eSIM involves activating a mobile network on a SIM card already present in the iPhone for phone calls ...

Open the Settings app on your iPhone ... You should also ensure that the inserted SIM card works with your iPhone, especially if you transferred it from another device. Update your iPhone 4.

your SIM card slot is possibly malfunctioning. It's best to just switch to an eSIM if your iPhone supports it. To know whether you have an eSIM-compatible device, navigate to Settings > Cellular.

Basically, carrier settings contain packets of data about ... potentially a necessary step if you insert a new SIM card into your iPhone, for instance — it's a quick process.