Protocols & Standards
SIP (Session Initiation Protocol)
By Myroslav Orshak · operates licensed carrier infrastructure · Updated September 2026
SIP (Session Initiation Protocol) is the standard that internet phones use to start, change and end a call — it does the negotiating, and something else entirely carries the audio.
How it works
Think of SIP as the conversation before the conversation. When you dial, your phone sends a SIP message to the other end saying who is calling, who is wanted, and what kinds of audio it can handle. The far end replies that it is ringing, then that it has answered. Both sides now agree how to talk, and only then does the audio begin.
The audio itself is not carried by SIP. That job belongs to a separate protocol, RTP, which streams the sound directly between the two ends. This split matters far more than it sounds, because it explains the strangest and most common VoIP fault: a call that connects and rings perfectly but has no sound. SIP did its job and RTP could not find its way through.
Why it matters
SIP is why you are not locked to one vendor. Because it is an open standard rather than one company's invention, a phone from one manufacturer works with a service from another, and moving providers does not mean replacing hardware. When a provider says a phone is compatible, this is the standard they mean.
It is also worth knowing the name because it appears in every diagnosis. A router feature called SIP ALG breaks calls by rewriting these messages badly. Phones losing service are said to have lost SIP registration. And the split between signaling and audio is the first thing a support engineer will reason about when only one side can hear.
SIP is defined in IETF RFC 3261, published in 2002 and still the current specification. Its universality is why any SIP phone works with any SIP provider, and why the phone market has never fragmented into incompatible ecosystems.
Related terms
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What is SIP in simple terms?
It is the language phones use to arrange a call: who is calling, who is being called, and what audio formats both can handle. Once that is agreed, the actual sound travels separately. Setting up the call and carrying it are two different jobs done by two different standards.
Is SIP the same as VoIP?
No, though the words get used interchangeably. VoIP is the general idea of calls over the internet; SIP is the specific standard almost everyone uses to set those calls up. Nearly all business VoIP is SIP-based, which is why the terms have blurred together.
Why do calls connect but have no audio?
Because SIP succeeded and the audio stream failed. The signaling found its way through your router while the separate audio stream did not, usually because of SIP ALG, double NAT, or a firewall closing the return path. It is the most common VoIP fault and it is almost always inside the building.
Do I need to understand SIP to buy a phone system?
Not at all. It matters in exactly two situations: checking that a phone you already own will work with a new provider, and understanding a support engineer when something breaks. Everything else about choosing a system is about price, features and support.