Unlocked phone

You can't add a new carrier unless your phone is currently unlocked.

Your next question might be how do you know your phone is locked to your current carrier.

Tap Settings > General > About > Carrier Lock

If you see SIM Locked, your phone is locked to the carrier that you are using now.

lf you are planning to travel to the Caribbean, it might be tons easier to simply add an international option to your current plan with your current carrier.

If you're planning to purchase an iPhone 14 in the US, you need to keep in mind that it is eSIM only. There is no physical SIM slot. Even if you purchase an unlocked phone, you will not be able to use it unless the carrier supports eSIM on the iPhone.