Got a prepaid phone with SIM, Do I need to activate it to use overseas?

Hey all,

I heading to Australia for Christmas and I just purchased a Prepaid T-Mobile Quad Band phone, Nokia X2, to use while I'm there.

I'm familiar with the "buy a SIM chip in the country you're in, throw it in the phone and your g2g" idea, but I've only used unlocked phones that have been activated already.

My question is, can I just take an unlocked phone fresh out of the box, take it to Australia, or any country for that matter, and just get a SIM chip in it and I'll be good to make calls and text messages.

OR

do I need to do the entire: activate this phone with T-Mobile, select my plan, pay for my minutes, so that I can simple have the phone activated initially, then I'll be okay to use in a different country.