This article may help you to understand the SIP call flow during a OWA call answering rule call forwarding scenario. I have a SNOM PBX , exchange 2013 and SFB 2015/Lync 2013 mixed deployment.
One of the user has OWA call forwarding rule configured to transfer the call to a external PSTN endpoint when you press 1. User is enabled with 4 digit UM SIP URI based dialplan.
Call was initiated from a PSTN endpoint to a Lync user and it was connected to voice mail. You can see the call flow in below article.
User was connected to voice mail and entered DTMF digit 1 to transfer the call to sales department. UM server has created a refer after verifying the certificate and message identity attribute has been added to the invite.
Lync FE server would route the call only if message identity attribute is inserted to the invite as both endpoints are PSTN and we can not assign a voice policy/PSTN usage to external PSTN endpoints. Following is the example for REFER identify header,
1222 1186 0221EAB8 Debug 2018/09/27-01:22:36.881 21548 19596 MSExchangeUM CallSession (35777208) : Call-ID e3e0ef1f-5236-428f-99be-bfe258d1168e : ReferredByHeaderHandler::ParseHeader() SIP header 'Referred-By': <sip:test2@>;ms-identity="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:Thu, 27 Sep 2018 17:22:24 GMT";ms-identity-info="sip:LYNCSE1.:5061;transport=tls";ms-identity-alg=rsa-sha1
You would observe 404 and call will be routed to edge if message integrity attribute is missing in the REFER invite. UM server extra traces will show DFTM input and REFER header creation as below. Once it is completed, call will be blind transferred to the destination and disposed from UM server. S4/SIP , inbound outbound call stacks stack and UM extra traces will be helpful for troubleshooting.
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