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Tom Arbuthnot MCSM Communications

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What Happens with #Lync phone sign in if I have duplicate Extensions?

Published 24/03/2013 - 1 Comment


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This question was posed by and tested by my friend Jed Ellerby (worth following), he tested on Lync Server 2010 and I verified on Lync Server 2013.

Lync Phone edition phones can be signed in by entering your extension and Pin (it can also by signed in via USB tethering on the phones that have USB). Your extension is defined in Lync by appending “;ext=XXX” on SIP Tel URI, e.g. if my extension was 1234, my TelURI might be tel:+442034201234;ext=1234

I can then go up to a phone and type 1234 and my pin (set my administrator or at “”) and sign into a Lync phone edition phone.

But what happens if I have two offices with overlapping extension ranges? Their TelURI can still be legal and globally unique, but their extensions clash? e.g.

Michael in office 1 is:


John in office 2 is:


It would be best, if possible, to have non-overlapping extension ranges. However when replacing existing systems, existing dialling habits and/or company acquisitions it’s quite possible to end up with overlap. How does Lync Phone Edition handle it?



Lync server signs them in correctly, as long as their pin is unique



Michael’s pin is 147258

John’s pin is 258369


What happens if the extension and pin are the same?


The GUI/PowerShell does not do any validation check for duplicates, but if users try to sign in they will see a fail


Pins are usually randomised and 6 digit, so the odds of hitting a clash are slim, but it is something to look out for, especially if you do have known overlapping extension ranges. In an ideal world you would move to a unique extensions globally for easier dialling and management.

Update: Jed found that Microsoft to reference this behavior:

“The Front End Server uses the combination of full phone number or extension, and PIN, to uniquely map enterprise users to their Active Directory credentials”


Tom Arbuthnot

Tom Arbuthnot

Principal Solutions Architect at Modality Systems
Tom Arbuthnot is Principal Solutions Architect at Unified Communications specialist Modality Systems. He is a Microsoft Certified Master and MVP, blogger, regular on The UC Architects Podcast, and speaker at events including Microsoft TechEd and Ignite. He co-runs The Microsoft UC User Group London.

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