Power down the phone, then replace the old SIM card with a SIM from a different network. Check that the iPhone is able to connect a phone call over the new network. That’s the short version ...

only the iPhone 14, 15 and 16 models sold in the US are eSIM only. This means you cannot insert a nano-SIM card into these devices. However, you can have multiple eSIMs installed on these devices.

Here’s an outstanding hack that adds a dual SIM slot to a US-sold eSIM iPhone 15/15 Pro ... you can likely install this mod for yourself. Of course, you lose some things, like waterproofing ...