Skype4B Server 2015: SIP/2.0 503 Service Unavailable

While working on a PSTN integration with Skype for Business Server 2015, we notice that the inbound calls were being rejected immediately with SIP/2.0 503 Service Unavailable:

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TL_INFO(TF_PROTOCOL) [festd\festd]6984.23A4::03/06/2017-22:56:29.167.0000245B (S4,SipMessage.DataLoggingHelper:sipmessage.cs(801)) [4223311279]
>>>>>>>>>>>>Outgoing SipMessage c=[<SipTcpConnection_1ACDDA8>], 172.20.20.40:5068->172.20.15.39:56945
SIP/2.0 503 Service Unavailable
FROM: “Joule Adams”<sip:09920156101;phone-context=PrivateUnknown@recore.lab;user=phone>;tag=f0d14b5566;epid=8CE36BBBAB
TO: <sip:+449920207101@172.20.20.40;user=phone>;tag=9837f16c5d;epid=14123C3787
CSEQ: 12538 INVITE
CALL-ID: 49bfd2ea-8753-4651-a8aa-d7b48d066d93
VIA: SIP/2.0/TCP 172.20.15.39:56945;branch=z9hG4bKdab2e468
CONTENT-LENGTH: 0
SERVER: RTCC/6.0.0.0 MediationServer

The related SIP messages didn’t had any information why the Mediation Service immediately rejected the call.

However, in the Trace tab we found an error related to this call:

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TL_ERROR(TF_COMPONENT) [festd\festd]6984.5B3C::03/06/2017-22:56:29.075.00002454 (MediationServer,ProxyCall.ProcessGatewayCallStateChanged:proxycall.cs(1446)) [25728949]
25728949Exception: System.MissingMethodException: Method not found: ‘Boolean Microsoft.Rtc.Internal.Collaboration.Media.SdpMediaDescription.ValidateMediaDescription()’.
at Microsoft.RTC.MediationServerCore.ProxySdpProcessor.InternalTryParseSdp[TGlobalDescription,TMediaDescription](SdpContentDescription cd, ProxyOfferAnswerInfo& offerAnswer, String& parsingError)
at Microsoft.RTC.MediationServerCore.ProxySdpProcessor.TryParseOffer(SdpContentDescription offer, Boolean bypassOffer)
at Microsoft.RTC.MediationServerCore.ProxyCall.ProcessGatewayCallStateChanged(Object sender, CallChangeEventArgs args)NULL

TL_INFO(TF_PROTOCOL) [festd\festd]6984.5B3C::03/06/2017-22:56:29.168.0000245C (MediationServer,ProxyCall.TerminateSession:proxycall.cs(3822)) [25728949]
25728949Terminating Proxy side call with session state: Idle, sip-response: 503, ms-diag: 10014, ms-diag reason: An internal exception received while processing the incoming request.NULL

This happened because the Enterprise Voice and Dialin Conferencing features were added after the Front End server update:

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Then we update these components with the Skype for Business Server 2015 Update Installer:

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Finally the inbound call will work without issues:

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