Take the following situation.
1.) I create a company offering "premium" jokes.
2.) I do a Google search for South African phone numbers, find 25,000 of them (Google is amazing

) and send them all the following SMS:
"We have located this number, and have decided that someone such as yourself would love to receive a daily joke! If you do not wish to receive these, SMS "NO" to *My R50 SMS Shortcode Number Here*"
3.) I wait a day
4.) Anyone who SMS'd "NO" gets charged R50 (Profit already - Woo hoo

), and anyone who didn't I add to my "Premium" joke-offering service list.
5.) Every day, I send a Google a 50 character joke, and send it to everyone on the list (With the above opt out message to keep things legal). I bill their phone companies R100 for these SMS's informing them that they chose to receive them (They didn't say No, did they? That means they want them!)
6.) Profit! \o/
On an unrelated note, the person who said banks "cant" sell your details is lying. Sure, they might get fined, arrested, etc - But they still can if they want to...