Toll fraud is the theft of long-distance service by an unknown third party. The 'thieves' can be 'call sell' operators. These are groups that steal and sell long-distance for profit. They can also be 'phone phreaks' who are hackers that do it for entertainment and sometimes for profit.
However, whether they do it for profit or fun, the effects can be devastating. Some businesses have been hit to the tune of $25,000 to $30,000 by fraudulent long-distance charges in a single weekend.