Comment by pmoriarty

I was walking around San Francisco the other day and suddenly noticed that my phone had switched from its usual AT&T provider to "China something" (maybe "ChinaNet"? I don't remember exactly what it was, and it was only there briefly). And then I got a text saying:

  Welcome to the test network.  Your IMSI is IMSI:  346234543252301

Not very stealth in this case. And I'm still not sure what it was. It went back to AT&T after I walked around some more.