Microsoft Teams Calling is a powerful extension to the meetings, collaboration and teamwork capabilities already built in to Microsoft Teams, which brings the ability to assign phone numbers to people and teams, and allows many organisations to finally retire their ageing phone system, or PBX.  But if your organisation has established a communication strategy based on Microsoft Teams, how smooth is the process to bring your existing phone numbers with you?  Let’s find out by exploring how the process works today, and some potential pitfalls to avoid.


There are two primary methods of bringing phone numbers into Microsoft Teams by connecting it with the Public Switched Telephone Network (or PSTN):

Direct Routing

You leave your phone numbers, trunks and PSTN services exactly where they are, and connect them with Microsoft Teams using a certified Session Border Controller.  This might reside within your own data centres, or it might be managed or hosted by a service provider.

Calling Plans

Let a service provider do all the difficult integration work for you.  Microsoft offer their own Calling Plans in most markets within North America and Europe, or by partners in other regions.  Instead of paying for trunks and infrastructure, you pay for Calling Plans on a subscription basis – per user per month.

Direct Routing
has been available globally since Teams Calling was launched.  It provides flexibility and a BYO option for organizations with a need to co-exist with other phone system platforms.  With this option, you don’t need to port telephone numbers into Microsoft Teams Calling, you continue to manage these with your current Telco instead.
More information: Plan Direct Routing (Microsoft Docs)
When talking about porting phone numbers into Microsoft Teams Calling, we’re talking about Calling Plans.  While Microsoft initially set about becoming a Telco themselves in the regions where this is fairly straightforward (North America and Western Europe) two things soon became clear:
  1. In most countries, the public telephone network is highly regulated, if not state-owned – which makes it really difficult for a new provider like Microsoft to become a Telco operator
  2. Customers and Service Providers alike want choice and flexibility, and the ability to innovate by connecting Microsoft Teams Calling to existing services, without having to incur the complexities, support and security responsibilities of a Direct Routing setup.

The answer to this is the announcement about Operator Connect at Microsoft Ignite in March earlier this year, that will launch and reach General Availability later this year.  But as early as 2018, Microsoft has been laying the foundations for Operator Connect by partnering with Telco’s like Telstra in Australia, and later, Softbank in Japan, to build a framework for Calling Plans to be delivered by existing service providers.

Porting versus Migrating

Just as many of the Microsoft Docs articles about phone number porting are written largely from a U.S.-centric perspective, this article equally talks from an Australia-centric perspective, with respect to the Telstra Calling for Office 365 calling plan product for Microsoft Teams – however it’s highly relevant beyond Australian shores, as the Telstra calling plan product is the pre-cursor to Microsoft’s new Operator Connect framework being launched globally.

To clarify a point of terminology:

  • Porting of telephone numbers typically refers to moving these from one carrier, or operator, to another.
  • Migrating on the other hand, typically refers to keeping these with the same carrier, and simply moving these from one service to another.

For example, if your telephone services and your number ranges are already with one of the carriers above, and you want to use them with Operator Connect when it launches, you could simply Migrate those number ranges to the new Operator Connect service with that carrier.

If on the other hand, your number ranges are with another carrier in Australia, and you wanted to use these with Telstra’s calling plan service, you would be Porting these number ranges between carriers.

What does the Porting process look like today?

Porting number ranges into Microsoft Teams Calling in Australia is fairly straightforward, and is driven by the new, or destination carrier.  The following steps will occur:

Step 1: Submit the request

The customer submits a request with the new carrier to port in an existing number range.  The request will include a Customer Authority Form to authorize the incumbent carrier to participate in the porting process, and release the number range to the new carrier.  You can specify which phone numbers you want to use as Service numbers, and which numbers should be User or Subscriber phone numbers.

Step 2: Validation

The incumbent carrier will perform pre-port validation checks:

  • Does the customer listed in the CAF form actually own the telephone number(s) listed?  Is the billing information correct?
  • Are there any other services attached to the telephone number(s) that will be impacted by porting them out?

Within Australia, the ACMA set forth a 7 business day SLA for this step.

Step 3: Interim Cutover

The telephone numbers being ported in are provisioned into the Office 365 tenant.  At this point, phone numbers can be allocated to users and call flows, and can be used for outbound calls.  However, inbound calls are unchanged, and will still route via the incumbent carrier until Final cutover (Step 6 below).

Step 4: Batch order

Once the incumbent carrier confirms the numbers can be ported, the new carrier will create a porting batch order, which is then sent back to the incumbent carrier for approval.

Within Australia, the ACMA set forth a 7 business day SLA for this step.

Step 5: Cutover appointment

Once the port order is approved, the new carrier contacts the customer to book a commissioning, or cutover appointment.  Typically there is a 10 business day lead time for booking commissioning appointments, and restrictions or additional costs may apply for after-hours cutovers.

Step 6: Final cutover

The commissioning appointment is typically 2 hours in duration, and all of the numbers or number ranges listed in the batch order will be ported during this window.  The customer telephone systems admin must also be present to test that the numbers are working with the new carrier – in Australia, this is an ACMA requirement to ensure that essential services, such as emergency calling, are not impacted.  If this testing is not successful, the change will be rolled back and the routing changes made during the porting process will be reverted.

While a
Migration of telephone numbers between services within the same carrier can often be completed in a matter of days, Porting of number ranges will typically take around 5 weeks as per the above steps, and can be longer when carriers are dealing with a backlog due to resource shortages.  A lot of this will be streamlined and automated under the new Operator Connect framework, and customers will be able to initiate this process directly within the Office 365 Admin Portal.

It’s important to note also, that numbers in 100-number ranges (such as 02 1234 5500 through 5599) cannot be separated or broken up in any way.  You cannot port half of a 100-number range, or even a single number within that range, while leaving the rest of the range with the previous carrier.  In general, that rule also applies to migrations within the same carrier, although some carriers allow you to share number ranges between different services offered by that carrier.

What is this “Interim cutover” step all about?

You’ll notice one step is highlighted in the table above – Interim cutover.  While it may go by different names in different places, it refers to the practise where phone numbers appear in Teams Admin Centre in your Office 365 tenant ready for allocation before they are actually cut over and working for inbound calls.

So – if you’re porting or migrating your number ranges all at once from a legacy ISDN or SIP trunk service connected to your dusty old phone system, Interim cutover is great!  It allows you to start testing (outbound calls only) and more importantly, to start provisioning your users and call flows with numbers, so you’re not having to do that step during the Final cutover commissioning appointment.  However, there are a couple of scenarios where Interim cutover will cause issues, and must therefore be skipped.

The effect of Interim cutover on migrations from Direct Routing

If you are already making telephone calls in Microsoft Teams using Direct Routing, and you are migrating or porting your number ranges into Microsoft Teams Calling Plan, you’ll need flag when you submit your porting or migration request to skip the Interim cutover step.  This is because while Direct Routing phone numbers are not managed in Office 365 in the same way as Calling Plan phone numbers, when Interim cutover occurs, all your Direct Routing phone numbers are loaded into your Office 365 tenant as Calling Plan phone numbers in preparation for Final cutover, and can then be viewed via the  Get-CsOnlineTelephoneNumber cmdlet in the Teams Powershell module:


As these phone numbers are now being treated as Calling Plan numbers, Microsoft Teams will attempt to route them as such, and users who are assigned these phone numbers in the current Direct Routing setup aren’t able to make outbound calls.


  • You can allocate Calling Plans to your Direct Routing users before you submit the telephone number porting request to ensure outbound calling works as expected during the Interim cutover phase.
    • Caveat:  officially, Microsoft don’t support “trunk-to-trunk” calling scenarios in Microsoft 365 Phone System, so you may run into issues with call forwarding or transfers back out to the PSTN, which ingress via one trunk (Direct Routing) and egress via another (Calling Plan).
  • The recommendation in this scenario is to flag to the new carrier that Interim cutover should be skipped.  Your phone numbers will be only loaded into Office 365 during the Final cutover appointment, and will need to be provisioned to your users and call flows within that migration window, or shortly thereafter.

The effect of Interim cutover on site-by-site migrations

While it may be feasible for smaller organizations, or those with only a handful of sites to migrate all their telephone numbers at once, most larger organizations will migrate each site individually.  Consider a typical migration site where a large organization with many sites is migrating to Microsoft Teams Calling.  At some point in the project, some sites (e.g. “Site 1”) will be successfully migrated to Teams Calling, while other sites (“Site 2 and 3”) have yet to be migrated, and are still using the previous phone system:


The old phone system could be a dusty old PBX connecting to the PSTN via ISDN trunks, it could be a newer IP phone system connected via SIP, or a single telephone at a retail store connected to an individual business line.  It could even be an on-premises deployment of Skype for Business with Enterprise Voice, in all cases Interim cutover will impact the site-to-site call routing.

The reason for this is that when the telephone number ranges for “Site 2” are placed into Interim cutover state and loaded into the Office 365 tenant, Teams Calling already in use at “Site 1” will view those telephone numbers as internal to the Microsoft Teams stack.  Therefore, when a user at “Site 1” is calling the telephone number of a user at “Site 2”, Teams will resolve that telephone number as being an internal call within the Office 365 tenant, rather than routing the call to the PSTN, and the call will fail:



  • It’s strongly recommended to craft your user adoption and training material to encourage staff to use dial-by-name to make a Teams-to-Teams call when calling someone internally.  That’s because the call can then be escalated into a video call, and you can also send messages during the call, for example if you need to send someone a link to a document or web page.  Teams-to-Teams calls are unaffected by Interim cutover because they don’t use telephone numbers at all.
  • If that’s not a viable approach for your migration, you’ll again need to flag to the new carrier that Interim cutover should be skipped.  Your phone numbers will only be loaded into Office 365 during the Final cutover appointment, and will need to be provisioned to your users and call flows within that migration window, or shortly thereafter.


The ability for staff to be able to make and receive phone calls from any location using any device makes Microsoft Teams Calling a compelling user experience, especially during times when staff need to remain connected and productive while working from remote locations.  The ability to bring your existing telephone numbers into Teams Calling allows organizations to switch to Teams Calling without disruption to the business and their customers and contacts – however, careful migration planning is required, as well as a solid understanding of call routing during co-existence scenarios, to ensure a smooth and seamless transition.