How does the "Give up their phone number" scam work? How do you "[bill] $20 a month" based on that?

Do I need to be even more paranoid about filling out web forms than I thought?

Edit: It seems it's the same mechanism that allows things like Collect calls to be billed to the receiving party. So I guess it has legitimate uses- there just needs to be a better way to keep it from being abused, apparently. http://www.fcc.gov/cgb/consumerfacts/cramming.html