"It's worth noting that since VoIP is a digital emulation of a system that functions, in part, on old analog rules, it can be attacked in both modes. The POTS (plain old telephone system) was vulnerable to attack many ways, for example, through "phreaking," as in the classic story of the toy whistle that was packaged in boxes of Cap'n Crunch cereal that could emit a tone that would put the phone in operator mode, at which point the caller could call anywhere.
"There also are VoIP-specific protocols, such as the ultra-important SIP (session initiation protocol) architecture, which can have their own vulnerabilities. These protocols transport over IP networks, typically in UDP packets, so certain classic networking vulnerabilities may come into play. VoIP hardware devices, too, may have their own peculiar vulnerabilities"
How safe is VoIP?