Source note · Call transfer semantics REFER and Replaces

In a semi-attended transfer the Transferor ends its attempt to reach the Transfer Target before that session is established and proceeds with the transfer anyway, the Transferor's user agent continuing as an attended transfer after the Transferor hangs up; RFC 5589 states that media must be played to the Transfer Target on answer or the Target may hang up and the transfer operation will fail.

Checked 2026-09-16

Note that media must be played to the Transfer Target upon answer -- otherwise, the Target may hang up and the resulting transfer operation will fail.

Vendor: IETF
Product: Session Initiation Protocol
Subsystem: Semi-attended transfer
Deployment: not restricted
Region: not restricted
Release range: BCP 149
Checked: 2026-09-16

Sources

RFC 5589 — Session Initiation Protocol (SIP) Call Control - Transfer — IETF / RFC Editor · tier 1 standards and regulators · RFC 5589 section 7.6 Semi-Attended Transfer

Cite this note

APA

WarmTransfer. (2026, September 16). Call transfer semantics REFER and Replaces: source note xfersem-semi-attended-needs-media-on-answer. WarmTransfer. https://warmtransfer.net/knowledge/claims/xfersem-semi-attended-needs-media-on-answer

BibTeX

@misc{warmtransfer-claim-xfersem-semi-attended-needs-media-on-answer,
  title  = {Call transfer semantics REFER and Replaces: source note xfersem-semi-attended-needs-media-on-answer},
  author = {{WarmTransfer}},
  year   = {2026},
  url    = {https://warmtransfer.net/knowledge/claims/xfersem-semi-attended-needs-media-on-answer},
  note   = {Source note xfersem-semi-attended-needs-media-on-answer, checked 2026-09-16}
}

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