I would have thought that in the UK T-Mobile have a legal obligation to block numbers which call your phone.
If you live in the UK check out
Home page | Ofcom (not sure of the alternative in the US), anyway find out the info you want and state it to T-Mobile, if they still don't comply then complain to ofcom:
Problems with your mobile phone | Ofcom
Hope that helps.
But a mobile phone blocker app would be great, at least it would be a hell of a lot easier than having to ring us customer service each time you want to block a number.