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Guide: How to Migrate Tenant to Tenant Telephone Numbers in Office 365

Microsoft Teams has grown massively in the last 12 months, growing from 44 million active users in April 2020 to over 145 Million users in April 2021. But the business world doesn’t stop; mergers, acquisitions, divestitures is part of the everyday challenge for the IT Pro.

Voice and how we use voice in Microsoft Teams is also very much part of the “modern office” in which many of us live today.  One aspect we are going to review today is how do we look at moving numbers from tenant to tenant. Microsoft believes this is a relatively straightforward task, but there are plenty of things that we need be mindful of should we want this to be seamless.

The first task we need to do is to have the information required.

Source Tenant domain name

To find the source Tenant domain name:

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  1. Log in to your Office 365 Admin Center as an administrator.
  2. Under Setup click on Domains (or just click here).
  3. Find the domain that ends with .onmicrosoft.com , this is your Office 365 tenant name.

Source tenant unique identifier

The tenant unique identifier is often known as a globally unique identifier or GUID.  To find this from within the Azure AD admin center:

Your tenant ID can be found in the Tenant ID box on the Properties page.


Destination Tenant name and a unique identifier

We also need this same information for the destination tenant, follow the above steps to get the Office 365 destination tenant domain name, the Office 365 destination tenant unique identifier.


The Number(s) which are to be ported from source to destination tenant.  These need to be broken down as a user number or a service number.  User numbers are applied to a user.  Service numbers are applied to Auto attendants, call queues or virtual numbers which need to follow a particular function but are not attributed to a physical user.


The addresses on the destination tenant for user numbers will need to be supplied.

To find out the used addresses, use Teams Admin Centre and go to Locations and Emergency Addresses. This will output the list of all the addresses currently in use. These addresses will need to be added back into the Teams Admin Center from the source to the destination Emergency addresses.

In addition to this, the locations must also then be validated. Each location that a user could possibly call from will need to be added to this list of locations. If your company has more than one address, it’s likely that your organization will require more than one emergency location. If the location shows as validated, it is then possible for emergency services to be sent to the address you’ve set. It’s worth noting that if you call 112,911, or 999 (whichever is applicable from your location) that the emergency services will be called from this emergency location and not from your physical location. This means if your emergency location is in London, however, you’re working from Paris, the location will be from the UK and not France.

The system should look similar to this example with addresses and the locations, the number of numbers allocated to each of the locations and if the site is validated.


To create the emergency locations, click on New, then input the information. Should you have the Emergency Location Identification Number (ELIN) input it, however, this is optional. The ELIN is when you are dividing the emergency addresses into locations or places, you can have more than one ELIN.

Once Completed press Save, Microsoft will validate the address, and the address will be added to.

This validation will need to be completed before the numbers can be assigned to the tenant.

Request date and time

The last thing Microsoft will require is when you are looking to have the port completed.  This will only be done Monday-Friday (no national or bank holidays) and it’s 8-5pm UK time for Europe and the same is applicable for the United States.

Once you have compiled this information, this will need to be sent to Microsoft for further review and validation.  Once this is completed, they will then email you that the request is successful or if further review is required.

To email the European Union or United Kingdom Porting team use: [email protected]

To email the United States or Canada use [email protected]

A useful template of this can be found here:


Once the numbers have been provisioned to your tenant, they will be ready to be assigned.  Numbers that have not yet been validated or ingested into the tenant properly can still be assigned ahead of the porting completion.  In addition to this, all users who have a telephone number from Microsoft will require the correct licenses to do so.  This will always include Teams, Phone System, and in this instance a calling plan, as Microsoft are providing the calling services.


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Microsoft is hard-wired into Jason’s DNA - Jason Wynn is a Principal Technology Strategist at Content and 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.

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