So, I looked at this list: http://en.wikipedia.org/wiki/List_of_SMS_gateways
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.
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 ;(