Teams IP Phone: External call forward drops – Replaces Header 420 Bad Extension

Already a few weeks ago I noticed that Microsoft Teams has changed its behavior. Microsoft sets a replaces header as a prerequisite for redirects to the office. However, some providers like Vodafone or Telekom do not like this. A desktop client does not send this replaces header. According to Microsoft this is a requirement for a long time, but I could not understand this in all implemented projects.

Phone System Direct Routing – Microsoft Teams | Microsoft Docs

---- Incoming SIP Message from 52.114.75.24:1025 to SIPInterface #6 (Teams) TLS TO(#147) SocketID(1263) ---- [Time:11-06@12:22:42.290]

INVITE sip:+49XXX:5061;user=phone;transport=tls SIP/2.0
FROM: <sip:+49XXX@sip.pstnhub.microsoft.com:5061;user=phone>;tag=0045a6334d29449c90ca2af48227949c
TO: <sip:+49@sbc01..de:5061;user=phone>
CSEQ: 1 INVITE
CALL-ID: d7a281ccce025658a513542f3f8e5314
MAX-FORWARDS: 70
VIA: SIP/2.0/TLS 52.114.75.24:5061;branch=z9hG4bK96423ed9
RECORD-ROUTE: <sip:sip-du-a-eu.pstnhub.microsoft.com:5061;transport=tls;lr>
CONTACT: <sip:api-du-b-euwe.pstnhub.microsoft.com:443;x-i=33d7fff9-ff71-4ca1-b98a-96cfd3183393;x-c=d7a281ccce025658a513542f3f8e5314/d/8/208af177202840e4b849e3771ff38695>
CONTENT-LENGTH: 1103
MIN-SE: 300
REFERRED-BY: <sip:+49XXX@sip.pstnhub.microsoft.com:5061;user=phone>
SUPPORTED: timer
USER-AGENT: Microsoft.PSTNHub.SIPProxy v.2021.5.28.7 i.EUWE.2
CONTENT-TYPE: application/sdp
ALLOW: INVITE,ACK,OPTIONS,CANCEL,BYE,NOTIFY
P-ASSERTED-IDENTITY: <tel:+49XXX>
PRIVACY: id
REQUIRE: replaces
REPLACES: cfd69ef0632c5b03b16a2b11b7d89b07;from-tag=1d57d6dbec6f4cafab58ead4002f8131;to-tag=789ef976
SESSION-EXPIRES: 3600

Telekom, for example, responds as follows:

12:22:43.000  ---- Incoming SIP Message from 217.0.129.229:5060 to SIPInterface #1 (SIP-Trunk DSL) TCP TO(#185) SocketID(5) ---- [Time:11-06@12:22:42.371]

SIP/2.0 420 Bad Extension
Via: SIP/2.0/TCP 80:5060;branch=z9hG4bKac969524204;alias
To: <sip:+49XXX@sip-trunk.telekom.de;user=phone>;tag=62fe3f43
From: <sip:+49XXX@sip-trunk.telekom.de:5061;user=phone>;tag=1c1161367804
Call-ID: 4746484741162021122242
CSeq: 1 INVITE
Unsupported: replaces
Reason: TSSI;cause=4200001
Content-Length: 0

To solve the problem, the parameter on the IP profile of the provider is set as follows.

According to the manual, the parameters are described as follows.

[0] Standard = (Default) The SIP UA supports INVITE
messages containing Replaces headers. The device
forwards the INVITE message containing the Replaces
header to the SIP UA. The device may change the value
of the Replaces header to reflect the call identifiers of

■ [1] Handle Locally = The SIP UA does not support
INVITE messages containing Replaces headers. The
device terminates the received INVITE containing the
Replaces header and establishes a new call between
the SIP UA and the new call party. It then disconnects
the call with the initial call party, by sending it a SIP BYE
request.

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