(See update to the problem below)
I have a couple of cases with customers, which are using Lync 2013 together with a Sip trunk provider, and the Call Forwarding don’t Work for them.
The Setup is as the following:
UserA (external user) calls UserB (Lync User). UserB has sim-ring to an external number (UserC – a mobile phone for example). The call will be rejected by the sip trunk provider.
A trace with wireshark looks like below:
The trace shows that the SIP invite to the Sip trunk provider only includes the A-Number and the C-Numberr, and not the number of the user, which has made the call forwarding (B-Number).
Because the sip trunk provider verifiers the call by a number they trust (in this case the B-number) to make sure that it’s not a spoofed call, the invite gets rejected, because the A-number and C-number in this case don’t belong the customers sip trunk.
Back in the old days (so last year), when we all were using Lync 2010, we had the possibility to add the “Referred-By” header to the invite from the mediation server to the sip trunk.
This is not possible in Lync 2013 in the same way.
After created a case with Microsoft they confirmed that the invite do not contain all the information when using call forwarding.
A workaround could be to install a 2010 mediation server and implement the “Referred-By” solution, however it turns out that there is another bug in 2013 Frontend server so it don’t send the “ms-referrer” info to the 2010 mediation server. The result is that the “Referred-By” do not work on the 2010 mediation server, because it does not have the info on who has made the forward.
The status right now is that MS do not have a workaround and is working on a fix. They could not give me a fix date, but said properly be fixed in a future update.
It’s not all sip trunk provider which are affected by these problems.
I was quite disappointed that CU1 didn’t solved my problem with call forwarding, however I did some more testing and actually found a workaround to the problem.
All my users are configured with the tel:+1234567890;ext=1234 format – but if I remove the ;ext=1234 then it works!!!
So keep the users line uri as tel:+1234567890 and together with the History info on the SIP trunk, it will forward the call history to you sip trunk provider on call forwarding and Sim-ring.
When you don’t have the ext on the users, this means that if you are using Lync hard phones you will need to login with the complete Line uri number.
The newly released CU3 fixes the problem.
Microsoft writes about it here
Get the CU3 update here