I've read various pages about buying a data roaming addon for my upcoming trip to the USA.
Some of the pages suggest that on arrival I'll get sent the usual text message welcoming me to whichever network BT have partnered with over there and offering me a text back code to buy the data bundle.
I don't have much faith in these kind of systems.
Can I not just buy the bolt-on now and be done with it?
If I do buy it now, I assume I don't start eating into that data allowance until I get there?