*This article has contributions by Mike Weaver, Senior Product Manager at Quest.  Mike specializes in Office 365 tenant-to-tenant migrations and provides a wealth of experience in mergers, acquisitions, and divestitures, or (MAD).

With the COVID-19 pandemic driving rapid adoption of Office 365, tenant-to-tenant migrations are becoming more common. Although a ton of content covering tenant-to-tenant migrations is available, little detail exists around how to move the Teams Phone system as part of a migration.

Phone system migration challenges

Phone system migrations can be challenging for multiple reasons. When purchasing phone systems and associated hardware, businesses buy enough capacity for the current needs plus expected growth. Organizations rarely know about major changes that might happen later, such as acquiring a company with other solutions in place. Such complications are part of the cost of merger, acquisition, and divestiture – or (MAD) projects.

Many phone solutions are incompatible with other systems, have significant feature differences and technical challenges. During a migration process, it is often best to choose one system to allow for optimal call handling. At the same time, telephony systems often include critical customer-facing workflows and processes which must remain up and running to preserve client relationships.

Migration paths

Migration options fall into some common categories:

  • Migrate it all – The gold standard is to move all users, data, and workflows over to the target system, but this is not always possible. In phone migration projects, complete migration is usually only possible when migrating between identical systems. Even then, it may not be possible to integrate things like call recordings. While this is the desired outcome, it’s not where phone migration technology is today.
  • Dial tone recoveryA dial tone recovery is when the migration process turns off the phone service in the source and turns it on in the target. No data migration occurs; the goal is to move functions (the phone numbers and routing) over to the target. This strategy is common in situations where data isn’t that important, or systems cannot be migrated due to compatibility issues or API limitations.
  • Offline data retention – This method is like a dial tone migration, but the data stays in another system. For phone migrations, this data includes call recordings and call quality scoring information. The old system might be kept online for an extended period, or the data might be migrated into an archive or eDiscovery system. This strategy is common when the source uses a very different system than the target, and the data is needed for performance reporting, legal needs, or other reasons. However, providing users with access to their data can be a steep challenge. In addition, for regulated companies, data retention may be a requirement with unbudgeted costs.


Divestitures often called “carve-outs,” are a significant challenge because companies rarely operate based on an expectation of splitting off a section of the business.

During migration planning, pay particular attention to the Transition Service Agreement (TSA). This agreement defines how long the source firm must provide service to ease the transition. A common frustration is for this agreement to include items that are not technically possible. For example, the TSA may state that users keep their existing phone numbers, and while this may seem simple, it can be quite difficult or even impossible.

In addition to the technical issues, divestitures can have a major business impact, with both the source and target firm making significant decisions that create extraordinarily complex or even impossible challenges for IT, especially under the tight deadlines typical in divesture scenarios. Therefore, it is paramount to involve IT professionals experienced in telephony in the decision process.

Moving Teams Phone

Now that we’ve covered the basic issues involved in moving phone systems during MAD projects, let’s consider the specifics involved with the Teams Phone system. Microsoft offers different ways of integrating Teams Voice with a private switched telephone network (PSTN), or traditional telephony.

  • Microsoft calling plans: Calling plans are included in some Microsoft 365 SKUs, or available as an add-on. Calling plans (available in domestic and international variants) allow Teams users to call PSTN landline and mobile numbers with no integration into any other service provider. Microsoft calling plans are currently available in 33 countries.
  • Direct routes: Direct routes allow organizations to use a session border controller (SBC) to access a SIP trunk. SBCs come in many flavors, such as a service provider providing the trunks and SBC, or attaching your own SBC to Teams. You might choose a direct route if you have a multi-year agreement in place and want to get the most out of that investment. Or you may find that you want to provide telephone services to a location that isn’t on the list yet. In that instance, that will be an excellent reason to utilize a direct route.

What happens if an organization then needs to move Teams Phone from one tenant to another?

  • If using a Microsoft calling plan, the project is straightforward, because the telephone service remains within Microsoft. From the Teams Admin Center, go to the “Phone Numbers” page and click the “Get phone number support” button. (More help on how to contact the porting teams can be found here.) This initiates the process of number porting. Required information includes your domain name, domain unique identifier or tenant ID, request date and time, the list of telephone numbers, and the emergency locations. During the porting process, service numbers (call queues, auto attendants) and users will be assigned to the telephone numbers.
  • For organizations utilizing a direct routing solution, the migration process is more technically difficult. If you change providers, the porting can take weeks or even months. In addition, you must move entire ranges of telephone numbers at a time; if you have phone numbers that are not part of a continuous chain of numbers (which is common), then you typically cannot move them.

Auto attendants and call queues

Auto attendants and call queues can be an essential part of Teams Phone. Many policies and procedures, both internal and external, can use these services. For instance, it is not uncommon for sales and helpdesk processes to depend on these capabilities. In a tenant-to-tenant migration, auto attendants and call queues need to be re-established in the target tenant after telephone numbers are moved to the target tenant.

You also need to be mindful of service numbers. Like shared mailboxes, service numbers are not tied to a particular user. They typically belong to a call queue or hunt group. Service numbers could be a number for a conference call number or an auto-attendant, also known as an interactive voice response (IVR). The restriction outlined above applies here: You cannot take a single number from one provider to another. However, most providers allow you to switch a telephone number to another customer provided you stay within that provider. This situation applies when both organizations in the migration use the same provider.

Channel-enabled call queues

Call queues can use a Microsoft 365 group or team to manage the agents assigned to the queue. In simple terms, a user belonging to a team and a particular channel within a team can be assigned to pick up a call queue. This feature simplifies managing the queue by using an interface that people already use. Leveraging Teams also removes the need to give users access to phone management interfaces, which can introduce risks and other issues. One issue with these third-party interfaces is that they often do not support limiting admin access.  Users who may need to manage a single call queue could make mistakes in other queues and configurations.

However, using groups for queue management adds complexity to a tenant-to-tenant migration, because we need to ensure that all the relevant people are moved from one channel to the next. If the list of members constantly changes, the team owners need to know when the list will “lock” and will likely need to make changes post-migration.

Furthermore, we must configure the call queues to use the same channel in the other tenant to ensure that no calls are lost along the way. This exercise can add considerable reporting to the tenant-to-tenant scoping process. As part of the planning stage, it is essential to ensure that all channel-enabled queues are discovered and set up in the target correctly.  During the planning process, communications need to be sent to the users managing the queues that include cut-offs for changes to membership.   

Operator Connect

Microsoft supports Operator Connect for Teams, which allows an organization to link seamlessly to a certified telephone operator. These operators allow you to maintain your contract, your relationship, and your cost per minute while linking them to Teams. This consistency is good news for organizations that want to move seamlessly into one organization. If you maintain your contract, you can move from tenant to tenant, re-ingest your existing telephone numbers back into Teams, and then reassign the telephone numbers appropriately.

However, there are significant issues when organizations move away from a voice provider in a tenant-to-tenant migration. In migration, the telephone provider or operator provides new numbers. The old numbers that were assigned must then be removed and added to the new tenant. The administrator can then go into the Teams Amin Center under Operator Connect and reassign the numbers in the target. This is a significant piece of work.

Teams’ phones and meeting rooms

A variety of phones are available. The certified providers currently include Yealink, Crestron, Lenovo, AudioCodes, and Poly. The full list of devices can be found here.

Teams room systems are also available from various manufacturers, along with solutions for managing that equipment. Many devices can be used to make and receive phone calls and can be assigned a PSTN number. It is important to use certified providers; the list of certified providers can be found here.

The Teams Admin Center can manage many aspects of Teams phones and rooms. Regardless of the manufacturer, all devices will need to be reregistered or authenticated to the new target tenant. Depending on the number of rooms and physical endpoints, this can be a significant undertaking. Unlike many phone system migrations, one thing that helps in this type of migration is that if a phone that works on one tenant, it will work on another.

That said, most phones will need to go through a hard reset; otherwise, the phone will not show in the Teams Admin Center. Currently, a phone that leaves one tenant and joins another without being reset cannot reregister to Intune in the target tenant, which will leave the phone being managed by Intune in the old tenant. A hard reset may not seem like much, but with many devices, it can take considerable time to hard reset, sign back into the handset and validate the phone is working as you expect. Do not underestimate this effort!


As with any migration activity, it’s a good idea to take your time and do some testing before making any decisions about how to proceed. You can sign up for a test tenant to help in this process – it pays to practice.

About the Author

Jason Wynn

Microsoft is hard-wired into Jason’s DNA - Jason Wynn is a Principal Tech Strategist at Content + Cloud in the United Kingdom. He is an Office Apps & Services MVP with nearly two decades of experience with Microsoft Intelligent Communications including Skype for Business, Microsoft Teams, and Office 365. In addition to his day job, Jason also runs Microsoft Cloud User Group in London and Birmingham, the annual Evolve Conference, a successful blog, and co-host of All About 365 Podcast. He loves sharing his knowledge with those inside the industry and covering anyone who hasn’t already seen the light.


  1. Tony

    Great article, very informative. Thanks for sharing!

    1. Jason Wynn

      Thank you, Tony, I’m glad you found it informative.

Leave a Reply