I remember getting all sorts of spam about very cheap conference calling. It can't be that expensive, we just set up a US number, the caller pays the long distance charges (which are very small right now)...
Well, we've had a tentative offer from someone to use their conference services; I'll see if that pans out.
However, conference calling services (one of which we use regularly) are ostensibly 'cheap', but their prices scale by the multiple of number of connections and length of time.
48 hours times many callers would shatter my budget.
Now, if you *own* the conference box (which is easy to do; you can buy 'em for a few K or build them easily), then you only have fixed costs, and can afford something like this easily. I believe that's the case in the service I've been offered.
Cheers,
Jeremy