To use a foreign service provider’s SIM card, you have to unlock your smartphone or buy a cheap unlocked phone. When you buy a new smartphone, it comes locked so you can use it with only one service provider. In case if your smart phone is locked but uses GSM system then you have to just talk to your service provider and activate your International roaming which comes with plans.
If you want to keep the same smart phone that is non GSM compatible then you have an option to buy a local phone with a SIM card. This will be affordable and save your money when you are overseas for a long period.
International SIM card allows you to switch between the international numbers without having to pay extra for your calls or chats. These calls also save your money and people who call you from other countries because incoming calls are mostly free.