What Does It Take to Track a Million Cell Phones?
Note that a lot of the information from the BTS is already available to anyone who "asks nicely".The mechanism that provides roaming is based on trust, so anyone connected to the SS7 network can query the location of any phone in the world and even intercept its calls. Just say to the home carrier "hey this phone is roaming on my network, would you be able to send me all of its calls and texts?".