How to convert a european iphone carrier to USA?

So I temporarily lived in Malta ( a small european island between Italy and Africa) and i bought an AT&T iphone there. I moved back to the USA and I want to make the phone american, if you can understand. Because when I use the maltese based carrier on the phone in the US, it'll charge me like $2 a minute for calling and etc. Sorry for the unclear question. So basically how can I convert to the american based AT&T and how much will it cost.

You need to make it unlocked which costs a few hundred.

To avoid HIGH charges you have to use a "domestic / national" simcard for the country you are in internally ( you know what I mean so don't think / say indoors!)

Using a "foreign / NON domestic" simcard means when you travel you are charged International rates

EG a Maltese simcard used in the USA, and you call a person who is physically 10 metres away
Your call is FIRST routed to Malta and then back to the US (BILLING).

THIS IS KNOWN AS "ROAMING", as outside Malta your phone would not even register in the US "Roaming" allows you to do that but is expensive as you pay for using the US network masts / infrastructure.

Same applies to me I'm Fair Close UK citizen, using any UK simcard in the US or any other NON UK country would eat into my credit

ROAMING again, my UK phone routes all my calls to the UK but I ask myself would it not be cheaper just to walk the 10 metres to have a conversation, as unlikely I would have their number!

Any unlocked phone allows uinsing any simcard in the phone.
Roaming means if I were in the US my Vodafone sim card would say show AT&T. If NOT Roaming I would have NO signal

I pay international rates if I use my domestic sim card (UK) in another country, I need to use a USA pre paid sim card.

The rates you pay in Malta are local rates for the sim card, cross a border and they become International.

You retain the credit but it cost more to use the phone