Microsoft Teams Phone number routing logic change, specific matching for users with extensions MC487014

Full message center message here: MC487014 (only sent to select tenants)

Quite a big change in how Microsoft Teams routes PSTN phone calls to users is coming. This is a change in behaviour that has been the same since the Lync server days.

This only affects users with an assigned phone number in the format where digits are added as an extension. There are two variants of this:

• The user has a unique DDI, but the extension is defined, e.g. +14025557334;ext=334
• A number of users are sharing the same base number; they do not have unique per-user DDI’s and just use extensions for routing e.g.
  • User Alice is assigned +14025557000;ext=111
  • User John is assigned +14025557000;ext=112

Using ext=xxx on phone numbers in Microsoft Teams is rare these days. Most users have unique DDI’s. I would not recommend they are used where possible.

However, if you migrated from Skype for Business, you might have used ext=xxx for Lync Phone edition extension and PIN sign-in (does anyone remember that?).

Current behaviour for Microsoft Teams extension routing

Today, if you have a user assigned a number in the format tel:+13305550100;ext=100 and you send a call into Teams for the number +13305550100, and no user is assigned +13305550100, it will match via Reverse Number Lookup to tel:+13305550100;ext=100  and the user will get the call.

In Direct Routing, the SBC sending in the call does not need to know if the user’s number assignment is in the format tel:+13305550100;ext=100 or tel:+13305550100, as the base number will get to the user anyway.

 

Change in routing behaviour: Calls will only route if there is an exact match with the user’s assigned number, including the ext=

After this change (due Feb/March 2023), if a user is assigned a number in the format tel:+13305550100;ext=100, the Session Border Controller\normalization rules must send the call to exactly that number “tel:+13305550100;ext=100” else the call will not route to that user.

Sending a call in the format +13305550100 will no longer route/match for the user assigned “tel:+13305550100;ext=100”

 

 

Actions for Teams Phone Admins

• Microsoft has used its telemetry to check tenants they believe have users with ext= format phone numbers assigned. If you have these, you should have received message center message MC487014, but it wouldn’t hurt to check your users. You could use PowerShell to scan your assigned phone numbers.
• I would recommend double-checking. Fabrizio Volpe has written a great little PowerShell script to check all your users here.
• If you have users in this format, and they have unique DDI’s, consider removing the ext=
• If you want or need to keep ext=, be sure that your inbound routing/SBCs/Direct Routing is all set up to send in calls that explicitly match that format.

If you are using extensions with a single shared base number, you should already have normalisation rules that take the short digit dial and map to the full E.164 number with the extension.

 

One challenge here is that Direct Routing as Service providers may not know how their customers have their user numbers formatted, and previously did not need to worry about this.

If you are a DRaaS provider, I would reach out to your customers and check if they have any ext= users. It’s quite possible they may miss or not understand the MC487014 message center message.