I've been trying to access the voice settings to modem-dial for a while, but I didn't think to (ab)use SMS this way.
Why T-Mobile only, though? In theory, you should be able to use this to talk to any SMS gateway, but you're going to end up paying on the gateway side ;(
Of the four major US providers, T-Mobile and Verizon are the only ones with MMS gateways (I use MMS to send the responses) that don't require a sign in by a customer (I only have T-Mobile myself). I tested Verizon's MMS gateway with my friend who has Verizon cell service, but it didn't seem to work for some reason.