VoIP is a key component in todays flexible, low cost infrastructure that is supporting business agility and growth. The biggest problem associated with VoIP today is so called toll fraud or more to the point call-jacking. A hacking team sets up a number of premium rate lines; gains access to an unsecured VoIP network; and sets up automated dial-ups. Over the course of a weekend, such attacks have been shown to leave the SME with a bill which can run into the 10,000s.”]
Source: https://informationsecuritybuzz.com/articles/unsecured-voice-calls-leave-the-door-wide-open-2/