Try to put something like an answering machine, a hayes-modem with AT commands, Asterisk, whatever it takes to make sure you increase THEIR cost (the cost maybe monetary if they have no fixed landline costs or, other like human time, when they are on flat-fee) Likely, there will no shortage of such scams in the future, so you will get to re-use your equipment and effort.
Make sure you engage them otherwise, if everybody strays away from this, they have more chances to continue the scam.
btw. the Google Voice suggested by others is the right course/direction of action, if you have live business phone traffic coming in; otherwise, you risk your clients. (and if it is such the case, $20 is a no-brainer to better serve your customers)