Thanks for the extra detail, rivviepop! Do you know anything about using the BB mail services while overseas? Do they stay on if you are on WorldClass roaming? Do they stay on if you have a prepaid sim (I doubt it, but maybe)?
I suppose if your BB mail works while on Tmobile, you could always buy a local SIM, and use it for phone calls, and once a day, pop your Tmo SIM card in and check your mail... The OP seemed concerned about his ability not just to make calls, but to respond to business email.
BTW, if you want to get your phone unlocked, Tmobile is pretty nice about it. You need to be a customer, in good standing, for more than 90 days, and you can unlock one phone per line every 90 days. Just call them, let them know you are travelling internationally, and making a more extended stay somewhere, and that you intend to buy a local SIM card. You might get transferred a few times, to the department that handles unlocking. They'll probably try to sell you on the WorldClass roaming, which is fine, have them activate it (since its free, and will work as an emergency fallback before you get a local sim), but have them send the unlock code anyway. They'll ask for your IMEI number, so make sure you write it down before hand (find it in Options>Status, or under your battery). The code'll hit your email box in a day or two, and includes instructions on how to unlock the phone. I think they are the friendliest phone unlockers in the biz.
peace,
sam