If you call some of the 118 services (a variety exist, with a very wide range of prices) then, for an appallingly high fee, they will connect the call for you to the number they've given. The fee is often in two parts - one just for having their computer dial the number, and one which depends on the length of the call.
Can they listen into the call? Dunno.
I can't see how they'd enforce it either. If you don't pay up for x% of the calls you get, you get dropped from the list?