FYI, I noticed a few text messages from the UK on my T-Mobile USA bill. I wasn't sure where it came from, but when I did a Google search of the number, I came across some old threads. Apparently, FaceTime attempts to activate by sending and receiving a text from the UK (why there, I don't know).
When I first inserted a T-Mobile USA SIM in my iPhone 4SI recall getting a funny message on my iPhone about how my carrier might charge for texts for se when I inserted a T-Mobile SIM for the first time, but didn't realize it was going to be an international text. Anyway, it looks like you can avoid this by disabling FaceTime before inserting a T-Mobile USA SIM.
When I first inserted a T-Mobile USA SIM in my iPhone 4SI recall getting a funny message on my iPhone about how my carrier might charge for texts for se when I inserted a T-Mobile SIM for the first time, but didn't realize it was going to be an international text. Anyway, it looks like you can avoid this by disabling FaceTime before inserting a T-Mobile USA SIM.