I configured this feature for a customer recently and whilst I have bestowed the benefits of deploying an “upstream” gateway and allowing it to route all calls based on AD look-ups for a while, when you deploy it and see it working, you really understand the power of these devices and how simple they make your DDI migration to Lync voice.

So what’s the issue?

When you migrate a user from the PBX to Lync voice, how do you ensure that calls bound for their DDI go to Lync rather than the PBX?

Possible solutions and pro’s and con’s

You could add an entry in the PBX routing table to route calls to a particular DDI to the SIP connection where Lync is.


  1. Provides a solution that incurs minimal change to the existing telephony environment.


  1. Not a strategic solution
  2. Routing table can quickly become unwieldy and difficult to manage. There can be limitations on the number of entries in the routing table and a large routing table in the PBX can have an adverse effect on PBX performance.

You could create a range of “ghost numbers” on the PBX which all route to Lync. You then configure each Lync user with a “Ghost number” as their Enterprise Voice DDI and “Call Forward” from the User’s PBX DDI to the “Ghost number”.


  1. Provides a solution that incurs minimal change to the existing telephony environment.
  2. No unwieldy PBX routing table
  3. Easy for a user to set up
  4. Can be a good solution for small numbers of users (eg small pilot)


  1. Not a strategic solution – it does not scale
  2. Inbound calls to Lync rely on the availability of the PBX.
  3. Call forwards and assigning new numbers can be difficult to manage

So each of these options are good for a small trial and to get a trial off the ground but neither of them is scalable nor strategic.

What is recommended as a strategic solution?

Deploying a gateway in an “upstream” configuration (between the PSTN and the PBX) and connecting it to AD allows the routing decision to be made based on information stored in AD. It is simple logic; as the call comes in from the PSTN is it for a Lync user? – Yes – Route to Lync, No – Route to PBX.

Migration of user’s DDI’s happens when you enable the user for Enterprise Voice in Lync. No PBX configuration is required and there are no routing tables or call-forwards to maintain.

It was excellent being able to configure Lync and the gateway to allow this functionality. I can see this type of deployment becoming more and more prevalent as people migrate to Lync voice.

A well configured telephony gateway (with well configured Lync of course!) makes call routing between your Lync environment, any legacy PBX’s and the PSTN a breeze.