A cautionary tale from 2010 :
Edwin Andrew Pena pleaded guilty to two felonies in connection with the hacking spree, Pena and cohort Robert Moore were accused of carrying out an elaborate scheme that routed more than 10 million minutes of VoIP calls over the networks of a dozen or so telecommunications providers without their permission. They breached the networks by using brute-force attacks that deduced the security telephone prefixes needed to gain access. To disguise the source of the attacks, the pair rerouted them through the computers of third parties.
Because the scheme piggybacked off the resources of others, virtually all the revenue was profit. As a result, Pena was able to sell long-distance calls for as low as four-tenths of a cent per minute, a fraction of what legitimate providers charged
This type of fraud is common place, piggybacking off your system, the bad news is, that whoever’s system they piggybacked off ended up with the bill, and the telephone companies would have charged you if you had been one of the unlucky ones.