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Tom Arbuthnot MCSM Communications

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Understanding Skype for Business Server and Online PSTN Conference ID Configuration

Published 25/01/2016 - 5 Comments

In Skype for Business Server and Online (or Lync 2013 / Lync Online), there are some differences to the defaults and settings around Conference ID’s

First off, when we talk about Skype for Business there are two conference ID’s, and a per user join PIN

  • The alphanumeric conference ID in the URL (referred to as the Meeting URL for ease)
  • The numeric “PSTN” Conference ID (referred to as the PSTN Conference ID for ease)
  • Each user has a personal numeric PIN to authenticate themselves to the conference if joining via PSTN, mostly users don’t authenticate and join as anonymous on PSTN unless authentication is forced



The per user join PIN is assigned by the system by default, on Skype for Business Server it can be set/reset on the page after doing AD authentication:



Skype for Business Server

In Skype for Business Server (and Lync Server 2013). By default, users have a single static meeting URL and PSTN Conference ID (their “dedicated meeting space”, sometimes referred to as “public”), every meeting they create use the same URL and ID. This makes it really easy for users to join meetings and remember PSTN ID’s, but can be considered a security risk as anyone that knows the PIN could join/meetings can overlap. With Skype for Business having a roster of users on the call, people “camping” on the line is usually less of a concern.

Users can elect to create “new meeting space” (sometimes referred to as “private”) in their meeting options for particular meetings, which creates a dynamic one-time meeting URL and conference ID. So they may be configured for a static dedicated meeting space, but use new meeting spaces for important or sensitive meetings.


On Skype for Business Server, administrators can choose to have users meetings always default to a new unique meeting space.

In Set-CsMeetingConfiguration, you can define the following parameters

  • EnableAssignedConferenceType – Indicates whether users are allowed to schedule public meetings. With a static/dedicated meeting space, the conference ID and the meeting link remain consistent each time the meeting is held. With a private/new meeting space meeting, the conference ID and meeting link change from meeting to meeting. The default value is True.
  • AssignedConferenceTypeByDefault – Determines if new meetings will be the dedicated meeting space by default. Set this value to True to use static meetings by default; set this value to False to use new meeting spaces (private) by default. The default value is True.

PSTN ID’s typically start at 4 or 5 digits and can grow as high as 12 digits, growth can be influenced by the number of Conference Directories, but not precisely controlled.

You cannot set or choose a particular PSTN Conference ID

Skype for Business Online

With Skype for Business Online. PSTN conferencing is provided by Microsoft’s “PSTN Conferencing”, an add-on service from Microsoft where they provide the dial-in PSTN numbers and ID’s, or from an Audio Conferencing Provide (ACP), a third party Microsoft partner who is directly connected to the Microsoft service to provide this number. In these cases, the PSTN Conference ID is defined by the PSTN Conferencing provider (either Microsoft or the third party ACP). For third party ACP’s it is always static, and there is no option to have a “new meeting space” with a unique PSTN Conference ID. For Microsoft PSTN Calling the conference ID is dynamic by default. It can be set to static, but only at the tenant level affecting all users.



Interestingly, in Skype for Business Online, even though the PSTN Conference ID is static with ACP, and can be set to static globally with Microsoft PSTN Calling, the meeting URL is always dynamic. So click to join will always be unique.

A couple of blog posts from my colleague Justin Morris are also worth a look on this topic:

Understanding Conference Security in Lync Server 2010

Why You Should and Shouldn’t Configure Unique Conference IDs in Lync

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, has a regular podcast with UCToday at and is a regular speaker at events including Microsoft TechEd and Ignite. He co-runs The Microsoft UC User Group London.


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Dewalt Kotze - 01/03/2017 Reply

Hi Tom

Nice and a simple read, I have a question about configuring a 3rd Party ACP in the On-Prem environment in a Hybrid deployment.

Currently the Online tenant is making use of the 3rd party ACP for the local number list and announcement service. We want to configure this for the on-prem hosted users as well to provide a consistent experience for dial in conferencing between on-prem and online.

The Technet article say to enable federation between the on-prem edge and the ACP, but surely it can’t be that simple.

Any information will be appreciated.

    Tom Arbuthnot - 07/03/2017 Reply

    Hi Dewalt,

    You are right, No it’s not that simple, as far as I am aware ACP partners cannot offer the same number set (service) and bridge it from SFBO to SfBS. Some do have server plugins to allow their service to be used on SfB Server, but it would be a different service/number set/charging model.

    Interested to get that article corrected if you have a link.



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