This article highlights options to deal with unassigned numbers in a Teams Phone System deployment. Recently, Microsoft made an unassigned number feature available for configuration using Teams PowerShell. But that’s not the only option to cope with unassigned phone numbers in your Teams VoIP deployment.

Source: https://pixabay.com/de/photos/telefon-alt-baujahr-1955-bakelit-3594206/

What’s the problem with unassigned numbers?

If you want to make and receive phone calls you need a telephony service provider which gives you a range or telephone numbers for your organization’s employees to enable them for PSTN calling. By default you are asked how many numbers you need and can contract a sufficient phone number range, e.g. +49711555555555-000 to -999.

In many cases you don’t utilize 100% of your phone number range and you might have employees leaving the organization or other reasons why not all phone numbers are assigned. The most annoying caller experience is probably if someone wants to contact a phone number which is no longer assigned. By default this call is terminated and the caller usually get some announcement telling that the number is no longer assigned (in the background, in SIP, this causes a 404 Not Found error message). Each PSTN “source” provider, i.e. the PSTN provider of the caller is coping with the 404 Not Found to terminate the call. Well, that’s not so nice, is it? What could we do better with these calls ending up with a 404 Not Found?

Source: erik365.blog drawing by Erik, example (with the new Microsoft Whiteboard app)

Wait, wait, wait … but there are different Teams Calling options to bring PSTN calling capabilities to Microsoft Teams. Yeah, right but so? Totally right, there are Direct Routing, Operator Connect and Calling Plans. You can cope with unassigned numbers differently or not depending on the deployment. E.g. in a Teams Direct Routing deployment you could utilize your SBC to handle the 404. That was my standard approach. I don’t explain this approach via SBC 404 handling here but add some links for further reading. Hereinafter, I like to give you an overview and example on how to deal with unassigned numbers in Teams natively.

Teams Unassigned Numbers

What does the unassigned number feature provide? (February 2022)

  • route to a custom announcement
  • route to the the main switchboard (user, resource account of an Auto Attendant or Call Queue)
  • configuration using Teams PowerShell 2.5.1+
  • no configuration using Teams Admin Center (February 2022)
  • if calling plans subscriber number –> communication credits required
  • if calling plans service number –> at least one phone system – virtual user license required

Routing a call via unassigned number solely to an announcement service which ends the call is nothing I would like to do. That’s just extra effort for no real added value in my opinion. And announcement is played by the PSTN “source” provider anyway as mentioned above. So let’s go for the second option, take a look at it and route a call to an Auto Attendant which you might have already for your main number (at a site, for a phone number range).

#Create new unassigned number treatment
#Route a call to an auto attendant's resource account
#Requires Teams PowreShell 2.5.1+
#Example

#Connect 
Connect-MicrosoftTeams
 
#Configure unassigned number to go to the main number's auto attendant for the site Stuttgart, BW, DE
$RAObjectId = (Get-CsOnlineApplicationInstance -Identity eriksautoattendant@domain.tld).ObjectId
New-CsTeamsUnassignedNumberTreatment -Identity UnassignedRangeSiteSuttgart -Pattern "^(\+49711555555555\d{3})$" -TargetType ResourceAccount -Target $RAObjectId -TreatmentPriority 1
 
#Disconnect
Disconnect-MicrosoftTeams

Conclusion, opinion and summary

This feature was overdue in my opinion especially for non-Direct Routing deployments wherein no SBC is in place to cope with unassigned numbers a bit more professional. I would like to see the option in the Teams Admin Center as well because smaller organizations leveraging calling plans might not have a full-time IT administrator who can spin up a Teams Shell and configure it. On the other hand they might have a Microsoft partner at hand who can support configuring this as needed.

To me, the feature for configuring Teams unassigned number handling is valuable. It can help to ease Direct Routing deployments a little because it simplifies the SBC deployment because there won’t be a need to configure unassigned number handling on a SBC. Furthermore non-Direct Routing deployments can benefit of this in particular because before the feature’s release there was no option in place to handle unassigned numbers.

Additional resources