I go to Europe on Occasion for work. I always ask T-mobile to unlock my phone because I will be traveling abroad. They are very willing to unload a phone if you have been with them at least 90 days.
I ALWAYS buy pre-paid sim cards for my phone when in Europe (tons of vendors sell them like here in the USA..every pharmacy sells prepaid sim cards). I remove my monthly sim and use the pre-paid ones I buy in Europe. T-mobile is pretty big in Europe and actually their prices are NOT bad for International traveling but I keep better track of my minutes if I just keep purchasing sims with a certain minute value on them....cheaper and easier.
When I go to Europe
