I have never actually used it personally, and it may be too difficult to integrate into a startup, but to get things rolling you might try looking at Frontline

I have never actually used it personally, and it may be too difficult to integrate into a startup, but to get things rolling you might try looking at Frontline SMS: http://www.frontlinesms.com/ http://www.frontlinesms.com/ Set it up yourself, get a couple of phones with unlimited texting packages and incoming texts will be paid for by the user. As I said don't have any experience actually working with it though and don't know how easy it is to integrate into a custom app.