Module 1: Foundations

Call Failures — CANCEL, Timeouts, and Error Responses

Learn about call failures — cancel, timeouts, and error responses

Duration: 7 minutes
Level: Advanced
Lesson: 47

Calls Fail More Often Than They Succeed

In a typical telecom environment, 30-40% of call attempts don't result in a completed conversation. That's not a bug — it's reality. People don't answer, lines are busy, numbers are wrong, networks hiccup. Understanding how calls fail in SIP is just as important as understanding how they succeed.

CANCEL: Aborting a Pending Call

CANCEL is how a caller says "never mind" after sending an INVITE but before receiving a final response. The most common scenario: the caller hangs up while the phone is ringing.

The CANCEL Flow

Alice                    Telnyx B2BUA                    Bob
  |------- INVITE ---------->|------- INVITE ------------>|
  |<------ 100 Trying -------|<------ 180 Ringing --------|
  |<------ 180 Ringing ------|                            |
  |                          |                            |
  |  (Alice hangs up)        |                            |
  |------- CANCEL ---------->|------- CANCEL ------------>|
  |<------ 200 OK (CANCEL) --|<------ 200 OK (CANCEL) ---|
  |                          |<------ 487 Terminated -----|
  |                          |------- ACK (487) --------->|
  |<------ 487 Terminated ---|                            |
  |------- ACK (487) ------->|                            |

Key points:

  • CANCEL has the same Call-ID, From-tag, and To-tag as the INVITE — it targets a specific transaction
  • The 200 OK to CANCEL just means "I received your CANCEL" — it does not mean the call was successfully cancelled
  • The actual result is the 487 Request Terminated response to the original INVITE
  • You must still ACK the 487 (as with all final responses to INVITE)

The Critical Rule: CANCEL Timing

CANCEL can only be sent after a provisional response is received and before a final response arrives. Why?

  • Before any response: The INVITE might not have arrived yet. Sending CANCEL to something that doesn't exist is meaningless. The UAC should wait for at least a 100 Trying before sending CANCEL.
  • After final response: It's too late — the transaction is complete. If a 200 OK arrives, the call is answered. You must send ACK then BYE to end it (you can't un-answer a call with CANCEL).

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