If I won this phone, I'd use the credits to call my very close friend in Paris, France. I haven't talked to him since he moved. I'd also use the credits to call family in Canada and Italy.
I'd call my mom and say "hey mom I just won a awesome new smartphone from Rebtel and Android Central so I'm gifting you my Galaxy S II! Welcome to the smartphone world mom!"
I would call all the friends I made at the LIVESTRONG summit I attended two years ago (most of whom live overseas) and check up on them. I am going to do that anyways. But it would be cheaper to do it with free credits and a better phone!
I would stay calling military friends if I could then literally everyone I know, and people I didn't. Seriously that guy in Scotland right now at the bar drinking ... Totally calling that guy.
I would call my college teammates who live over seas during our holiday and summer breaks. We are roommates when and school and this would be a great way to keep in touch.
I would use the credits to call my grandparents. They live in Georgia (I'm in Ohio) and since I see them only once a year I call them a lot and talk to them about what I'm doing in my life.