I actually tried starting a company based on that, but when I went to talk to the cell phone providers, one of them candidly explained to them that SMS was their cash cow and that it was essentially free for them, and calls were very cheap related to what the person paid, so it makes no sense for them to enable people to use the credit for other stuff which might get them into trouble later - think lawsuits, regulations, and all the complications of becoming a payment provider.