Which of the following options is the correct protocol used for phone calls…

Carrying live voice over a packet-switched network requires digitizing the analog speech, splitting it into small IP packets, and delivering those packets with the tight timing and low latency a real conversation needs. VoIP (Voice over Internet Protocol) is the umbrella technology and protocol family built around exactly this capability: the endpoints and codecs involved perform the actual digitisation and packetisation, while VoIP's signalling and transport protocols coordinate the call and carry the resulting stream. It is functionally distinct from protocols built only for link establishment, file transfer, or document retrieval.

Applying this to the four options: only VoIP is built around real-time voice transport in this way. A typical VoIP deployment uses a signalling protocol — commonly SIP or H.323 — to set up and tear down the call, and a media-transport protocol — commonly RTP over UDP — to carry the audio itself, though other signalling/transport combinations exist. That general mechanism is what is needed for a phone call to travel over the internet.

• PPP is a data-link layer protocol that establishes and authenticates a direct serial connection between two nodes (classically a dial-up link) and simply encapsulates other network-layer packets across that link — it defines a connection method, not an audio format or media stream.

• FTP is an application-layer protocol for uploading and downloading files between a client and a server, using a separate control connection and data connection — its job is bulk file transfer, not continuous two-way audio.

• HTTP is an application-layer request/response protocol used to fetch web resources such as pages and API responses — it is not itself a telephony or media-session protocol and defines no call-signalling or voice-transport mechanism, unlike VoIP.

Because only VoIP provides the real-time voice-transport capability a phone call needs, it is the protocol used for phone calls over the internet.