Microsoft Teams Update Affects Teams Voice

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Introduction

 

On December 17, 2022, Microsoft started to send MC487014 through the Message Center in the Microsoft 365 Admin Center to all tenants with users who have Teams phone numbers that have the extension “value” appended to their Teams phone numbers.

 

If you have at least one Teams user whose phone number looks like +14025557334;ext=7334, you should have received this notice in your Message Center or you will be receiving it. An update to this was sent on November 2, 2023.

 

If you have phone numbers that have the appended extension and you have ignored, or not seen this message, your Teams calling will fail!

The degree of the failure and the scope of your telephony outage depends on several factors. The change outlined in the message states that Microsoft was going to begin rolling it out in early July 2023 but pushed it back to mid-September anticipating completion of the rollout by mid-December 2023. In the past two (2) weeks we have had three (3) organizations suffer a Teams voice outage because they were not prepared for it. All three (3) organizations were unable to receive inbound calls from their customers, vendors, and associates for many hours. In one case it took nearly twenty (20) hours to get everything working!

* We encourage organizations that have Teams phone numbers with the appended extension to take this advisory seriously and remediate their Teams voice environment as soon as possible. *

 

Background

 

Basic Teams Call Normalization and Routing

  • When someone calls a Teams user from a non-Teams number such as a cell phone, Teams will try to match the dialed number to the LineURI of a Teams user in the organization. If Teams can find a match, it will route the call to the user. This is called Reverse Number Lookup (RNL). RNL has been used by the Microsoft Unified Communications products since the days of Office Communications Server (OCS) and continues to be used today by Skype for Business (SFB) and Microsoft Teams.
  • If the lookup fails, the call will fail. The call will not be routed to the Teams user.
  • If a Teams user tries to dial a phone number, Teams will try to normalize the phone number using the rules defined in the Teams Dial Plan. Normalize simply means to reformat into a common and consistent format. These numbers are typically normalized/formatted as 164 phone numbers.
    • For example, if a Teams user dials 4025551234, the Teams Dial Plan will normalize it to +14025551234. Teams will try to perform an RNL on this number and match it to the LineURI of another Teams user in the organization. If a match is found, the call will be routed to that user. If a match is not found, the call will be routed out of Teams to the Public Switched Telephony Network (PSTN) through a Teams Calling Plan, Operator Connect or Teams Direct routing.
    • If a Teams user dials an extension like 7334, Teams will normalize that number to +14025557334 or possibly +14025557334;ext=7334. Once again, Teams will try to match the normalized number to the LineURI of another Teams user in the organization. If a match is found, the call will be routed to the user and if not, it will be routed out to the PSTN.

 

What Is The Gist Of This Change That Microsoft Is Making?


In the past, if the Teams user John Doe had a LineURI of +14025557334;ext=7334 and Teams received a call with a destination number of +14025557334 or +14025557334;ext=7334 Teams would successfully match either of these normalized numbers to John Doe.

The change when applied to your tenant requires that normalized numbers exactly match the LineURIs of your Teams voice users and other voice-enabled entities like Resource Accounts, Dial-In Conference numbers, Shared Devices, etc.

  • If Teams attempts to perform the RNL on the normalized number +14025557334 and John Doe’s Line URI is +14025557334;ext=7334, it will not work, and the call will not be routed to John Doe.
  • If the dialed number presented to Teams is +14025557334;ext=7334, Teams will be able to successfully match the number to John Doe and the call will be delivered.

How Do You Prevent Teams Calling From Breaking When This Change Is Applied To Your Organization?


The simple answer is to make sure that the format of normalized numbers presented to Teams matches the format of the LineURI assigned to your Teams voice users, Resource Accounts, Dial-In Conference numbers, Shared Devices, etc. before Microsoft applies this change to your tenant.

The more detailed answer is that you need to ensure that there are normalization rules in your Teams Dial Plan that when applied to a dialed number correctly, format the LineURI of the Teams voice users, Resource Accounts, Dial-In Conference numbers, Shared Devices, etc. that you want to route calls to. 

Here are a few examples:

  • If you want to support dialing from one Teams user to another using a four (4) digit extension and your user’s LineURIs look like +14025557334, you will need to have a normalization rule in the Teams Dial Plan that has a pattern of ^(\d{4})$ and a translation of +1402555$1.
  • If you want to support dialing from one Teams user to another using a four (4) digit extension and your user’s LineURIs look like +14025557334;ext=7334, you will need to have a normalization rule in the Teams Dial Plan that has a pattern of ^(\d{4})$ and a translation of +1402555$1;ext=$1.

If your organization uses Teams Direct Routing, you need to ensure that manipulation rules (or whatever they are called on your SBC) are configured to normalize the dialed numbers to match the format of the LineURIs of your Teams voice users, Resource Accounts, Dial-In Conference numbers, Shared Devices, etc. You can also configure these manipulation rules on the Teams Online PSTN Gateway object representing the SBC, but we do not recommend it. 

Some examples:

  • If a call from the PSTN comes into your SBC over a SIP Trunk with a dialed number of 4025557334 or 14025557334 and the LineURI of the Teams user looks like +14025557334 the SBC will need to normalize the dialed number to +14025557334.
  • If a call from the PSTN comes into your SBC over a SIP Trunk with a dialed number of 4025557334 or 14025557334 and the LineURI of the Teams user looks like +14025557334;ext=7334 the SBC will need to normalize the dialed number to +14025557334;ext=7334.

It is very common for the LineURIs applied to the various Teams entities to have different formats. Your Teams user’s LineURIs may look like +14025557334;ext=7334 while the phone numbers associated with your Resource Accounts may look like +14025557000. In cases like this you will need Dial Plan normalization rules and SBC manipulation rules that accommodate both formats.


Summary

This change that Microsoft is applying to organizations who append an extension to their Teams user’s LineURIs are at risk of suffering a failure of Teams Voice. To prevent such a failure, organizations need to review their Teams Dial Plans and SBCs to ensure that normalization rules are in place to provide correctly formatted dialed phone numbers. The format of these numbers must match the formats of the organizations LineURIs or Teams calling will fail.


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