Module 1: Foundations

Call Transfer — REFER and Replaces

Learn about call transfer — refer and replaces

Duration: 7 minutes
Level: Advanced
Lesson: 48

Why Transfers Are Complex

Call transfer seems simple: move a call from one person to another. But in SIP's distributed architecture, it's surprisingly complex. There's no central switch that can just "reconnect the wires." Instead, transfer involves coordinating multiple independent SIP dialogs, each with their own state, and getting them to stitch together seamlessly.

Transfer is also the SIP feature most likely to break across different vendor implementations. The spec is clear, but edge cases abound.

Blind Transfer (Unattended Transfer)

Alice is talking to Bob. Alice wants to transfer Bob to Carol without talking to Carol first.

The REFER Method

Alice                    Telnyx B2BUA                    Bob
  |<======== RTP ==========>|<======== RTP ============>|
  |                          |                            |
  |------- REFER ----------->|                            |
  | Refer-To: carol@...      |                            |
  |<------ 202 Accepted -----|                            |
  |                          |                            |
  |                          |------- INVITE ------------>| Carol
  |                          |<------ 200 OK -------------|
  |                          |------- ACK --------------->|
  |                          |                            |
  |<------ NOTIFY -----------|                            |
  | (sipfrag: SIP/2.0 200)   |                            |
  |------- 200 OK (NOTIFY)->|                            |
  |                          |                            |
  |------- BYE ------------->|  (Alice disconnects)       |
  |<------ 200 OK -----------|                            |

Step by step:

  1. Alice sends REFER within the existing dialog to Telnyx, with Refer-To: <sip:carol@example.com> header
  2. Telnyx responds 202 Accepted — "I'll try to do what you asked"
  3. Telnyx creates a new INVITE to Carol (this is a new call leg)
  4. Telnyx bridges Bob's existing media to Carol's new media
  5. Telnyx sends NOTIFY to Alice reporting the outcome (using SIP fragment — the response code from the INVITE to Carol)
  6. Alice hangs up (BYE)

The NOTIFY messages use the "refer" event package — they carry SIP fragments that tell Alice whether the transfer succeeded or failed. The sipfrag body might contain SIP/2.0 180 Ringing (transfer in progress) or SIP/2.0 200 OK (transfer succeeded).

B2BUA Transfer Handling

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