Source note · Call transfer semantics REFER and Replaces
Where a Transfer Target does not support Replaces it rejects the attended transfer's INVITE with a 420 Bad Extension response, and RFC 5589's flow has the Transferor switch immediately from attended transfer to basic transfer.
Checked 2026-09-16
- Vendor
- IETF
- Product
- Session Initiation Protocol
- Subsystem
- 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 7.7 Attended Transfer Fallback to Basic Transfer and Figure 14
Cite this note
APA
WarmTransfer. (2026, September 16). Call transfer semantics REFER and Replaces: source note xfersem-attended-falls-back-on-420. WarmTransfer. https://warmtransfer.net/knowledge/claims/xfersem-attended-falls-back-on-420
BibTeX
@misc{warmtransfer-claim-xfersem-attended-falls-back-on-420,
title = {Call transfer semantics REFER and Replaces: source note xfersem-attended-falls-back-on-420},
author = {{WarmTransfer}},
year = {2026},
url = {https://warmtransfer.net/knowledge/claims/xfersem-attended-falls-back-on-420},
note = {Source note xfersem-attended-falls-back-on-420, checked 2026-09-16}
}