Module 1: Foundations

Basic Call Setup — INVITE to 200 OK to BYE

Learn about basic call setup — invite to 200 ok to bye

Duration: 8 minutes
Level: Advanced
Lesson: 46

The Most Important Call Flow

If you learn only one SIP call flow, this is it. The basic INVITE → 200 OK → ACK → media → BYE → 200 OK sequence is the foundation of every voice call. Every other call flow — transfers, conferences, holds — is a variation on this theme.

Step-by-Step: A Complete Call

Let's trace a call from Alice to Bob through Telnyx's B2BUA. There are two independent legs: Alice ↔ Telnyx (Leg A) and Telnyx ↔ Bob (Leg B).

Phase 1: Call Initiation

Alice                    Telnyx B2BUA                    Bob
  |                          |                            |
  |------- INVITE ---------->|                            |
  |<------ 100 Trying -------|                            |
  |                          |------- INVITE ------------>|
  |                          |<------ 100 Trying ---------|

Alice sends INVITE containing:

  • Request-URI: Bob's address (e.g., sip:+15551234567@sip.telnyx.com)
  • SDP body: Alice's codec offer and media address (where she wants to receive RTP)
  • From: Alice's identity
  • To: Bob's identity
  • Call-ID: Unique identifier for this call
  • Via: Alice's address for responses

Telnyx responds 100 Trying immediately — this stops Alice's INVITE retransmission timer. The 100 is hop-by-hop; it just means "I got it."

Telnyx creates a new INVITE on Leg B toward Bob. This is a different INVITE with a different Call-ID, different Via, and potentially different SDP (Telnyx's own media address, possibly different codec list). This is the B2BUA in action — two independent SIP dialogs.

Phase 2: Ringing and Early Media

Alice                    Telnyx B2BUA                    Bob
  |                          |                            |
  |                          |<------ 180 Ringing --------|
  |<------ 180 Ringing ------|                            |
  |                          |                            |

Bob's phone rings and sends 180 Ringing. Telnyx relays this to Alice (generating its own 180). Alice's phone now plays a local ringback tone.

Alternatively, Bob's side might send 183 Session Progress with SDP, establishing early media. In that case, the ringback tone comes from the network (through the RTP path) instead of being generated locally. This is how you hear carrier announcements or custom ringback tones.

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