Voice over IP (VoIP)
Carrying voice calls as packets over IP networks, with a signalling protocol such as SIP to set up calls and RTP to carry the media, sharing the data network's threats.
Voice over IP carries telephone calls as packets across an IP network instead of over dedicated circuit-switched lines. Signalling sets up, changes and ends the call, most commonly with the Session Initiation Protocol (RFC 3261). The media, the voice itself, travels in the Real-time Transport Protocol (RFC 3550), and its protected form is the Secure Real-time Transport Protocol (RFC 3711). VoIP is a familiar example of converged protocols: voice now shares the network, and the attacks, that data already faced.
The threats follow from that. Unencrypted media can be captured and replayed as audio; caller identity can be spoofed to support vishing; a compromised account or gateway can be used for toll fraud; and because calls are sensitive to latency and jitter, a modest denial of service can make them unusable long before a data service fails. Common controls are a separate voice VLAN, encryption of the signalling (SIP over TLS) and of the media (SRTP), strong authentication for handsets and quality of service to protect call quality.
Exam relevance: questions in this area tend to turn on keeping the two halves apart. Candidates are expected to know that SIP handles the call and RTP the voice, that SRTP protects the voice, and that availability matters as much as confidentiality for a service this sensitive to delay.