I do quite a bit of work in the mobile marketing field in the US, so perhaps I can shed some light on this.
SMS or text marketing is legal in the US. However, text spam is very illegal and there are stiff penalties carriers enforce against people who send unsolicited text messages.
Texts get a 100% open rate, which is why the are extremely effective --if (and only if) your recipient has requested the text. Double opt-ins are required for text campaigns. Also, you're required to provide instructions for how to stop the texts every two or three messages in a series.
SMS campaigns in the US are still very expensive because the sender has to pay and the receiver has to pay. But many companies find it's worthwhile.
You can find out more about how to ethically and legally use SMS marketing at the Mobile Marketing Association's Best Practices page here:
http://mmaglobal.com/policies
Twitter is an effective and free way to reach people by phone, if they want to be reached that way. As always, be relevant, useful and respectful of your reader's time.