Mobile phones are a media - for others than phone companies

Whenever my phone company has a great offer, they send me a text message. Neat!
But why don´t they sell this medium to other advertisers? It´s more personal than TV and print ads. And I wouldn´t mind receiving offers on products I know and use. To me, it´s obvious. The phone companies could combine it with a dedicated website, where you would be able to select which products are in your interest. And we could save a lot of time browsing several websites, clicking 'yes' to receiving offers.