A protocol is the shared set of rules (a communications convention) that devices follow when they exchange data. Just as human conversation carries unwritten rules - greet the other person, state your business, close with a farewell - communication between machines needs agreements that define what format data is sent in and how the other end replies. If one side follows its own private scheme, the line may be connected but no conversation happens.
The main protocol used in telephony is SIP (Session Initiation Protocol). SIP controls the setup, answering, and termination of a VoIP call, coordinating between devices the acts of placing a call, picking it up, and hanging up. The voice itself travels by RTP (Real-time Transport Protocol), and IP telephony works through the combination of the two. SIP settles the procedure for ringing while RTP carries the sound, and that division of labour is also the source of the trouble described below.
TCP/IP (Transmission Control Protocol / Internet Protocol) underpins all internet communication: IP handles addressing and TCP guarantees reliable delivery. HTTPS for web browsing, SMTP for email, and FTP for file transfer are all built on top of TCP/IP. RTP, however, runs over UDP rather than TCP, and UDP does not resend data that fails to arrive. In a conversation, a fragment of speech that arrives late is of no further use, so moving on beats retransmitting. That design is why a lost packet on an IP phone surfaces as a gap in the audio rather than a retry.
On the security side, TLS (Transport Layer Security) provides encryption. The S at the end of HTTPS stands for Secure and indicates that the HTTP exchange is wrapped in TLS, shielding what you send to and from a website from third parties. VoIP applies the same idea in two layers: SIP, the signalling, is protected with TLS, and RTP, the audio, with SRTP (Secure RTP). Protect only one of them and you get a half-measure - the number you dialled stays hidden while your voice travels in the clear, or the other way round.
What trips people up in practice is that each protocol takes its own path. Symptoms such as "the phone rings but audio only works one way" or "the call goes silent the instant it connects" are classic signs that SIP is getting through while RTP is being blocked by a router or firewall - not a faulty handset. The other pitfall is leaving unencrypted SIP reachable from outside your network: intruders take over the configuration and use the line as a springboard for expensive international calls. When you install a business phone system or an IP phone, always confirm that default passwords have been changed and that external access is restricted. Our guide to VoIP fundamentals shows these protocols at work.