If you are UK based I think they HAVE to pay at least minimum wage?
They don't mate, I'n not sure of the exact wording, but they have to disclose that 'there is every probability that you can earn at least minimum wage' or some such bollocks.
I used to do lots of price work, and this is how they get around having to pay a set wage.
There does have to be a very good chance you can earn minimum wage, and hoping people phone you is not that.
For price work, you are given a set task, and a price is agreed. For me, install the electrics for central heating, get 80 quid. The work was there, I didn't have to rely on people phoning me.
Big difference.