said by Smokey:
Its not that simple though... As you are not always on t-mobile's "network" even when your phone says that you are. I imagine that the best method to block texts would be a feature on the handset, and perhaps this is what t-mobile is saying they can not do. It would be great if someone knowledgeable in texting and GSM networks could chime in on a best possible method.
If you are roaming on a partner network, incoming text messages still pass through T-Mobile's network on some level, so I'm sure that wouldn't be an issue.
The handset solution is less than perfect. For example, if the customer is using an unlocked handset which is flashed with unbranded firmware, this wouldn't work. The user would have to flash their phone to the provider's version of firmware (if available for their specific model), possibly re-locking their handset and subsequently removing many of the features present on unbranded firmware which the provider sometimes removes.
This should really happen on the network level. If at&t/Cingular, which also operates a GSM network stateside, can block incoming text messages, there is no reason T-Mobile can't either. T-Mobile already has filters in place online, and really, how hard would it be for them to add an option to only receive T-Mobile system messages (e.g. your bill is past due, we have updated our terms and conditions, etc.)?