T-Mobile has such bad practices -- about 6 years ago they gave my phone number out as a temporary number to someone else. I don't know how their infrastructure is set up, but both me and this other guy had the same number for a time. Incoming calls would be routed to the phone that called out last. At one point I was able to talk to the other guy by using my wife's phone to call my own number. T-Mobile claimed that what was happening was impossible, so I filed a complaint with the FCC and switched my phone service. By the time T-Mobile responded to the complaint (by saying nothing was wrong), I had long since switched providers, so I didn't pursue the matter further. Huge annoyance though.