International Sim Card...Help?

I am going to Costa Rica next week, and I was wondering if I need an international sim card to use my phone (AT&T). If so, what will be the costs of calling/texting within Costa Rica (i.e. to others with me who are also using international sim cards) and back to the continental US?

My current plan includes unlimited texting. Will I be charged extra if I text in Costa Rica?

Also, if an international sim card is not mandatory, what is the purpose of an international sim card?

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There are two ways to use your phone in a foreign country. You can use your domestic carrier and roaming internationally in the foreign country. This is the least complicated but the more expensive choice. You pay international roaming rates for voice, messages and data. Incoming messages incur no extra charges.You use the SIM card currently in your phone but must have international roaming authorized for your line. People back home call you on your regular number. The other method is to use a SIM card from a carrier in the country you are visiting. You pay the domestic rates for that country. Your phone must be unlocked to accept a SIM card from a carrier in that country. You will receive a number from that carrier and people back home must be informed of the number and would pay international long distance charges to reach you. You would pay international long distance charges to reach them.

Go to these link to see your options for a carrier: Grupo ICE:ICE, Racsa, CNFL
Telefónica