(English) No PSTN Calls From SBA When WAN is Down

Recently I encountered a strange issue regarding PSTN calls initiated from a device on a remote site.

The environment consists of:
– Lync 2013 FE Standard
– Lync 2013 Edge
– Lync 2010 FE Standard
– Remote Sites with a NET SBA
– Remote Sites with a AudioCodes SBA

Now the Issue;
In ‘normal’ mode, Wan link was up, everything worked well, users were able to place PSTN calls.
In ‘NoUserServicesMode’ the Wan link was down, users were not able to place SOME calls, for example:
– calling service numbers worked
– calling mobile numbers failed
– calling some local numbers worked

We noticed that when a mobile call failed, sometimes the phone registered a call, resulting in a ‘missed call notification’ on the phone.

In the AC trace we could see the following happening:
Incoming SIP Message from SBA : INVITE
PSTN Send : PlaceCall
PSTN Recv : Call Proceeding
Incoming SIP Message from SBA : CANCEL

So, the SBA (Mediation) is sending a CANCEL to the GW, why? especially when you consider that in ‘normal’ (WAN is up) mode the same calls succeed.

After some research we discovered that this behavior also occurred in some Lync 2010 environments. The reason for this issue to occur has to do with the fact that the Mediation is expecting a ‘Trying’ within 10 seconds of the initial call setup. If not received the Mediation tries the next configured GW (if exists).

The timer responsible is the “FailOverTimeout” key in the “OutboundRouting.exe.config” file (C:Program FilesMicrosoft Lync Server 2013ServerCore)

Increase the FailOverTimeout to 20000 (20 seconds), save the file and reboot the SBA. After the adjustment of the timer all calls should succeed.

You can find the original 2010 post here:

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