Thanks so much for posting your question here to get help. I am sorry that we do not have the steps listed under the T989 already, but you can actually use this info: http://support.t-mobile.com/docs/DOC-2237
That article states that it is for the Galaxy S 4G (the little brother to our Galaxy S II), but the steps are exactly the same. I confirmed by comparing the steps there to the steps on my Galaxy S II.
Hope this helps, and have safe travels!
I would suggest making sure wifi calling works before you leave the states. Don't want to be embarrassed when you get somewhere and it doesn't.
You do need an updated SIM card [generally the one that came with your SII]. If it doesn't work due to your SIM card, just take it to a T-Mobile store and they'll replace it for free.