This is Location Based Routing Bug with External Users

Location-based routing is a relatively new addition to Lync 2013.  It wasn't part of the initial release, but the first cumulative update added this much asked for feature.

In a nutshell, location-based routing routes calls based on the network subnet the user is calling from, rather than their defined home server pool. So, if a user is in the Rome office today, all their calls can route via the Rome PSTN gateway.  If that same user goes to the London office tomorrow, their calls will route out the London PSTN gateway.

Since then, I've done a number of deployments where location-based routing was used extensively.  During one remote deployment at a large company, I noticed that my calls were not routing via my assigned voice policy.  They were routing via a PSTN gateway that was not defined in the policy I should have been using. In fact, my calls were routing out from South America (I'm in Canada)!

After much troubleshooting, I realized that Lync was routing my calls based on the subnet of my home network.  Turns out that my home network subnet of 192.168.2.x matched up with a corporate subnet assigned to that South American location. Location-based routing was configured to route calls for that subnet out through the South American PSTN gateway.  To verify this, I changed my home subnet to another one used by location-based routing. Lo and behold, my calls started routing out that PSTN gateway.  Finally, I changed my subnet to one not defined for location-based routing, and my calls began routing as per my assigned voice policy.

I wasn't using a VPN, and as such, I was connecting through the Lync edge server. Lync was incorrectly using my home's private subnet for call routing decisions. Since administrators have no control over the subnets used by external users, this could obviously lead to many issues, not to mention increased telephony charges for calls routing out through the wrong location.

I filed a bug report with Microsoft, who confirmed this bug and promised a fix in a soon-to-be-released cumulative update.  I don't know the details of the fix, but I imagine it will be one of two things:

1. Lync's behaviour will change to ignore private subnet information for external connections for the purposes of location-based routing. This is probably an easy fix, and my money's on this one. 
2. Lync's behaviour will change to use the detected public IP address assigned by the ISP for routing decisions.  I like this one, because it gives administrators the option to include public networks for location-based call routing decisions. Its unlikely that many administrators would go through the trouble to do this, but it would be nice to have the option.  

I doubt this bug will affect may deployments, but its always good to be aware.

UPDATE (08-Jan-2014): The issue has been fixed in the January 2014 Cumulative Update.  Read the KB article here.