The Tmo iBB plan is basically unlimited for the text portion of the message only. No attachments, no embedded graphics or you might wind up paying for data at roaming rates.
Tmo used to be fairly lax about billing data on the iBB plans until they realized they were losing who knows how much. Some people report they didn't get billed for data downloads and some do. At $15 a mb roaming rate you are taking a chance.
BB messenger is unlimited too.
Jim