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(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.


March Update

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.


October Update

The newly released CU3 fixes the problem.

Microsoft writes about it here

Get the CU3 update here

10 Responses to Bug in Lync 2013 Call Forwarding

  • Problem ist still present with july CU…. Can you share the case ID with me?

  • Hi Joachim. Can you confirm that the Problem is fixed? I cannot install the update soon.

    Thank you,


    • Hi Andreas

      Sorry for the late respons, I have just got the chance to install it at a customer today, which had the problem, and yes the problem is fixed now.
      The History-info now also contains the B number (the number that you originally dialed)


  • When using a 2013 mediation server and 2010 homed user the problem still exists regardless if they have an extension or not. No way to get the orginally called number which, as mentioned above, is required by some providers. Opened a case with MS and it was confirmed this is a bug.

  • call forwarding with exchange UM. When calling Lync to Lync call forwarding the tel:+18175551212;ext 1007 so we authenticate this way, but if we have a customer call into exchange 2013 UM the number does not come as tel:+18175551212;ext 1007 it comes as the inbound CLI or ANI of person calling into the system so no way to authenticate when the call is forwarding any solution

  • It looks, that is still problem with call forwarding in lync 2013
    I have updated lync 2013 and only call forwarding is not working.
    I try to enable or disable history-info but is not helping.

    User A call user B, B forward to C (mobile) and call failed.
    In network monitor I see that sip invite still use from: user A number instead of user B number.

    • Hi Asaf

      What dos the call fail with ?
      In the call forwarding it will use user A’s number in the invite – so you will see the right number on your mobile, and not your own Lync number.
      I have seen it before where the sip trunk provider didn’t allow that you manipulate the call id to user A’s number. The sip trunk provider was verifying the outbound calls though the call id, and rejected it because “user A” wasn’t a valid outbound calling number – Can you follow me?
      To solve this the sip trunk provider has to enable something called CLIP SA – not sure if that is a common name in the World or it is just in Denmark. But it allows calls on the sip trunks with any outbound numbers.
      Hope this answer helps you

  • Hi,
    session end with 480 No Routes Found at sip trunk provider.
    I already spoke with our sip trunk provider and the problem is becouse “sip re-invite session” do not contain a number of User B (neither in history-info or other headers), so he can not attach billing costs to the company that is making a forwarding and in that case the call is rejected.


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