I just read this blurb about a couple of guys that were sentenced in Germany for using Malware dialers with premium-rate numbers (976 sytle). They brought in $16m in 15 months.
There are several variations on this type of toll scam. From the article, it doesn't look like this one used Asterisk but it goes to show you how it could be used on the back end. And, it's a lesson to those who don't protect their boxes from toll fraud. If people like this can make calls from your box, they can drain your pockets at a rate of more than $15/line/min. http://www.itcinstitute.com/display.aspx?id=2855
