I wish this problem was exclusive to T Mobile, I really do. The sad thing is that I've been on two different networks and somehow firms seem to get hold of my mobile number and start calling me offering me an upgrade.
The most accurate firm was one who had my full Orange account details, so why wouldn't you trust a firm who knows where you live? When I reported this to Orange they acted surprised but did absolutely nothing about it, probably because data is flowing far too freely around their organisation.
My current provider isn't immune either, around 12 months on my previous contract with O2 I had multiple companies each trying to sell me a new contract. They claim it's just on an autodialer of numbers to call and have no personal information about me. However the fact that someone knows I'm on O2 means enough personal data is leaking.