You need to make sure you have international services active on your account prior to leaving the country. I would also ask the customer care agent you talk with to check what services are available in Guatemala. According to T-Mobile's website (http://www.t-mobile.com/international/RoamingMaps.aspx?tp=Inl_Tab_RoamWorldwide&tsp=Inl_Sub_RoamingMap&cc=GT&net=00&opts=0&cid=72 ) international internet is available on one carrier there but it doesn't state if BB's will have data access, only that GPRS is available there.
Zo