What is SIP?

SIP is the language phones use to set up, manage, and end a call. It does not carry your voice. It is the conversation that happens before, and around, the conversation.

The one-sentence version

When you make a modern phone call, something has to say "this person wants to call that person, are they there, will they pick up, and how should we send the audio?" That negotiation is SIP, the Session Initiation Protocol.

What SIP is not

Here is the part that surprises people: SIP does not carry your actual voice. It is only the signaling. Think of a phone call as two separate things happening at once:

• The signaling sets up the call: ringing, answering, hanging up. This is SIP.
• The media is your actual voice, sent separately once SIP has agreed on how.

It is like calling a restaurant to book a table. The phone call is the signaling, arranging the meeting. The dinner itself is the media. SIP books the table. It does not eat the meal.

What a SIP message looks like

SIP is text, and surprisingly readable. Here is the start of a real request to place a call:

INVITE sip:alice@example.com SIP/2.0
Via: SIP/2.0/UDP 203.0.113.10:5060
From: "Bob" <sip:bob@example.com>
To: "Alice" <sip:alice@example.com>
Call-ID: a84b4c76e66710
CSeq: 1 INVITE

That INVITE is Bob asking to start a call with Alice. The whole call is a series of these plain-text messages, back and forth, until someone hangs up.

Why it matters

Almost every internet-based call you make, from a WhatsApp call to a company phone system to a browser video chat, uses SIP or something very close to it under the hood. Once you can read a SIP message, you can see exactly where a call succeeded, failed, or got rejected, and why.