Microsoft is changing the Teams Phone number routing logic used for RNL (Reverse Number Lookup) for specific matching for users with extensions.

How does the Inbound/Outbound call routing work in Microsoft Teams?

When you receive a call from PSTN to Teams, the Teams check whether the number is assigned to anyone in the organization by looking at LineRUI attributes. If the LineURI finds the dialed number, the call will be routed to the destination user. Similarly, when the outbound call is made, it checks the LineURI using RNL (Reverse Number Lookup), so you don't need to route the internal call to PSTN and get it back e.g. Hair-Pin mode instead, the dialed number is checked and routed internally.

What is Reverse Number Lookup?

Reverse number lookup is a method of obtaining Caller information via their phone number from Active Directory.

You can update the phone number information in Azure Active Directory under Office Phone Number. In Skype, the AD information is synced and stored on the address book database, and each call is lookup against the LineURI to identify the caller name.

What's changes are coming to Microsoft Teams Call Routing?

There is a big change coming to Microsoft Teams how the called number is routed if you have your base number (10 digits) that also includes in your extension. 

There are two number patterns are impacted by this change,

• If you have a LineURI that includes your base number as an extension  e.g +13035557334;ext=334
• If you have configured Extension based dialing e.g,

•         User Alice is assigned +17205557000;ext=111
•         User John is assigned +17205557000;ext=112

Current Call Routing behavior with extension configured -

If you have a user configured their LineURI with tel:+13035557334;ext=334 and you dial a call into Teams for the number +13035557334 and NO user LineURI is configured, then it will do RNL lookup to tel:+13035557334;ext=334 and route the call to the destination.

Upcoming changes in the call routing -

Post Feb/March'2023, Microsoft Teams will accept only when the exact match with the user's assigned number, including the extension number. If the user number is assigned in the format of tel:+13035557334;ext=334 and the call is coming via direct routing, the SBC must be configured with a normalization rule to include the extension that exactly matches the destination configured on the user profile else, the call will not be accepted e.g +13035557334.

Teams Admin Action required -

To prevent any call failure, remove the extension number from user LineURI if a unique DID is assigned to the user.

If you want to keep the extension format, configure your inbound routing gateway to include the explicit routing format.

Which environment is not impacted?

If you are using Operator Connect or Calling Plan, this change will not impact you.

• Current Behavior:
• If a call is made to +13035557334;ext=334, we will find the user configured with the phone number +13035557334;ext=334
• If a call is made to +13035557334, and there is no user configured with +13035557334, we will find the user configured with the phone number +13035557334;ext=334.
• This can be an issue in situations where the same base number is used for multiple users.  For example: 
  User Alice is assigned +13035557000;ext=111
  
  User John is assigned +13035557000;ext=112
• If a call is made to +13035557000, and there is no user configured with +13035557000, we would match to the first user found with +13035557000 as a base number (Alice in this case).
• Upcoming change:
• Unless the SBC sends an exact match, the RNL service will not return a result.
• If SBC sends +13035557334
• User is configured with +13035557334;ext=334 => NO MATCH*
• *assuming no user is configured only with +13035557334
• If you have a unique LineURI configured for all users with tel:+13035557334;ext=720334, the RNL will find the MATCH, and the call will be routed.
• If the SBC sends +14025557334;ext=334
  
• User is configured with +14025557334;ext=334 => MATCH

Does the DRaaS environment impact?

If you are using Direct Routing as your operator's service, you should work with them to configure the normalization rule to match your user LineURI.