Charging "Per Text Message"

How does this work, exactly?

[Import Export]

How does the 'charging per text message' thing work? I'm talking from a business perspective, as if I wanted to start charging people for sending them something to their wireless device.

You see them all the time on late night TV and so on: Commercials offering ringtones, jokes, and images if you send a text-message to a special number. On the bottom of the screen you see that as a result of using this service, you will be billed on your wireless phone bill for $X.

How does this work from A-Z (from a business perspective)? I can't find any information on it..

Thanks :o)