I have used both ATT (old phone) and T-Mobile (currently) with my blackberries using BES. I have never been billed by either one for any data, in many countries around the world.

T-Mobile does allow you to just turn international email off and on whenever you want and pro-rates automatically. I believe ATT now requires you to keep the service for a year, or maybe two. This was not a requirement a few years back.

ALthough TMO says it is just email, I have not been billed for any other data. I do use Wifi whenever possible and take advantage of UMA (Wifi calling) whenever I can. THese calls (inbound and outbound) are just billed as if I am at home and count against my regular minutes. THere is a service for about $10/month if you want unlimited wifi calling, which would be for calls placed from anywhere via UMA.

One thing you might want to do when using wifi on TMO is to turn off the "mobile network" connection to be sure it doesn't switch over for some reason. I find this happens occasionally. Also be sure the "UMA" connection is showing (rather than EDGE, GSM, GPRS, etc) so you are not using the foreign mobile network, especially for phone calls!

And I automatically forward ALL calls to voicemail when I am out of the country to avoid people reaching me when I am roaming. More times than not it will be someone that has no idea where I am, or a wrong number, or the middle of the night.