I suppose you need a DNI for this, but my kids have a fixed monthly phone + sms pack of 65$, and above that I pay them 15$ per month for 200 MB data. That is enough for them. They don't mail all that much, but chat a lot, FB a lot, and call through 3G skype every weekend.
They are both very anxious to get the new Iphone (they both have the 1st 3G's that are more then 2 years old and are starting to fail, and of course, they want the new "gadget"
), and I hope I can do it with the plan they have now, and that I don't have to switch to the 189$ iphone plan.
I don't have a DNI, our phones are on the name of my husbands (argentine) socio.
When we go abroud, i switch of 3G and data roaming off, the kids take out their argentine card, and we all use our old Belgian SIM. I must recharge my phone before going because incoming phones abroud consume all my phone credit in no time. Then I have to call an argentine friend to go to the kiosko and buy more credit