So, I looked at this list: http://en.wikipedia.org/wiki/List_of_SMS_gateways http://en.wikipedia.org/wiki/List_of_SMS_gateways Of the four major US providers,

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