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by George Goglidze, CCIE #19926

Microsoft Teams DR & Avaya SBC - The missing ACK

Had a case recently with a customer that had an Avaya SBCE for Direct Routing with Microsoft Teams.

They had an issue with outbound calls to PSTN. Interestingly the inbound calls were working just fine.

After analysing the SIP Traces on the Avaya side, we have identified that the SIP ACK was not being received from Microsoft after the 200 OK sent, and after timeout 200 OK was resent, but the ACK was still missing.

This was basically what was happening:

 

MS Teams > SIP INVITE > Avaya SBC

Avaya SBC > 100 Trying > MS Teams

Avaya SBC > 180 Ringing > MS Teams

Avaya SBC > 200 OK > MS Teams

…….missing ACK from Teams here……

Avaya SBC > 200 OK > MS Teams

…….missing ACK from Teams here……

Avaya SBC > BYE > MS Teams

 

So after sending a200 OK a few times, Avaya SBC would give up and send a BYE message to terminate the call.

After some investigations, I have identified that the issue was because Avaya SBC was inserting itself into the SIP Record-Route of the 200 OK.

The 200 OK looked like this. I will not give the full details of the 200 OK, just the wrong part.

SIP/2.0 200 OK

Record-Route: <sip:sip-du-a-eu.pstnhub.microsoft.com:5061;transport=tls;lr>

Record-Route: <sip:sbcfqdnmasked.com:5061;ipcs-line=720908;lr;transport=tls>

Seems like Microsoft Teams did not like it for some reason. Once we have removed the second Record-Route where the SBC had inserted itself, everything started to work.

Is this correct behaviour? I do not think so. According to SIP Standards Avaya is in its full right to insert itself into the Record-Route 🙂

Unfortunately, Microsoft does not provide much information about this, as it’s not documented. But if you are implementing Avaya SBC, and having a similar issue, give us a call, we will be happy to resolve it for you 🙂

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