Teams phone numbers are ported, not moved

Everyone treats the voice piece of a tenant move like the mailboxes: point a tool at the source, watch it copy across, done by Friday. Then someone asks why the main line still rings in the old tenant, and you learn the hard way that a phone number is not an object you own — it is a service the carrier rents you, and it goes nowhere until a port order says so.

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This is part of the Azure tenant-to-tenant migration guide, and it is the section where the neat copy-everything mental model breaks. Users, groups, and mailboxes have a home you can read and rewrite. A phone number does not live in your tenant at all. Microsoft is explicit about it: telephone numbers aren't included with Teams Phone licensing, and the numbers in your tenant are acquired through the service provider that gives that tenant its connection to the Public Switched Telephone Network. The number belongs to a carrier and a PSTN service that are bound to the source tenant. The target tenant is, as far as the phone network is concerned, a stranger.

Why the numbers can't just move

A tenant-to-tenant tool copies data between two Microsoft 365 tenants. A phone number is not Microsoft 365 data. It sits in the carrier's records and in the national numbering system, mapped to the PSTN service attached to your old tenant. There is no field to flip that hands it to the new one. To bring a number across you either port it — a carrier transaction that reassigns the number to the service under the new tenant — or you give up on that number and acquire a new one in the target. Those are the only two doors, and both take real time. The comforting animation of a migration dashboard has nothing to do with either.

A phone number is a service the carrier rents you, not an object you own.

Port the number, or buy a new one

Porting is the right call when the number is public and printed on things — the main line, the support desk, the number on the vans and the invoices. You submit a port order and you sign a Letter of Authorization, the document that proves you have the right to move those numbers off the current carrier. Microsoft's porting wizard is unforgiving about detail: the billing telephone number, the account number, and the service address on that LOA have to match the losing carrier's records exactly, or the request is rejected and you are back at the start. You can port a whole account or just part of it, and you choose whether the billing number itself comes along or stays behind.

Acquiring new numbers is the right call for the rest — internal desks, a range nobody outside the company has memorized. You request numbers directly in the target tenant from its own provider, assign them, and update the directory. It is faster and cleaner than a port, and for numbers with no external footprint the fight to move the old ones simply is not worth it.

Which calling model you're on changes the mechanics

The target tenant needs its own PSTN connectivity before a single call connects, and Microsoft offers a few models that behave differently at migration time:

• Microsoft Calling Plans — Microsoft is your carrier. PSTN access, the numbers, and the port itself all run through Microsoft, which is the most self-contained path to run a port order into.
• Operator Connect — a certified third-party operator has already done the integration with Microsoft; you enable them on the target tenant and the numbers and PSTN service come from that operator. The port is a conversation with them, not only with Microsoft.
• Direct Routing — you connect any carrier through a Session Border Controller you own and run. Maximum control, and the most moving parts to stand up and re-point on the target side.

Decide the target model before you plan the cutover, because it dictates who owns the port order and how numbers get assigned once they land. Switching model and migrating tenant in the same weekend is two hard projects wearing one calendar entry.

Plan for the lead time, not the demo

Here is the number that governs the whole project: the port date is set by the losing carrier, not by you. Microsoft acknowledges a port request within 72 business hours, but the carrier on the other side sets the actual completion, and that runs from days to weeks once account and regulatory checks clear — longer across borders or when you are also changing providers. So porting is the long pole. Start the LOAs early, get the account details verified against the current carrier before you submit, and schedule the mailbox-and-identity cutover around the confirmed port date rather than hoping the numbers keep pace. If they slip, the phones ring in the old tenant while everything else lives in the new one, and that gap is exactly where a business notices a migration.

The takeaway

Voice is the part of a tenant move that refuses to behave like data. The numbers belong to a carrier tied to the old tenant, so you either port them — slowly, on the carrier's clock, with an LOA that has to be exact — or you buy fresh ones in the target. The calling model you land on decides who runs that port, and the auto attendants, call queues, emergency addresses, and policies get rebuilt on the far side from an export you were smart enough to take first. "Numbers are ported, not moved, and the voice apps are rebuilt" is the sentence that keeps the main line from ringing in a tenant you already left.

Questions people also ask

Can you transfer phone numbers between Microsoft 365 tenants?

Not by copying an object the way you would a mailbox. A number lives with the carrier and the PSTN service attached to the source tenant, so it reaches the target through a port order — a carrier transaction, authorized with a signed Letter of Authorization, that reassigns the number to the new tenant's service. If it isn't worth porting, you acquire fresh numbers in the target instead.

How long does it take to port phone numbers to Teams?

It depends on the carrier and country, not on you. Microsoft acknowledges a port request within 72 business hours, but the losing carrier sets the actual completion date, and that window ranges from days to weeks once regulatory and account checks are done. Treat porting as the long pole of the whole migration and start it early, with the cutover scheduled around the confirmed port date.

What is a Letter of Authorization for number porting?

A Letter of Authorization, or LOA, is the document that proves you are allowed to move the numbers away from the current carrier. It has to match the losing carrier's records exactly — billing telephone number, account number, and service address — or the port is rejected and you start over. Teams supports signing it by e-signature or uploading a signed copy.

Do auto attendants and call queues move in a tenant migration?

No. Auto attendants and call queues, and the resource accounts that hold their service numbers, are rebuilt in the target tenant by hand or by script. You recreate the greetings, business-hours and holiday schedules, call routing, music on hold, and agent groups, then assign the ported or newly acquired service numbers to the resource accounts once those numbers land.

Do you need to keep the same calling plan when you migrate tenants?

You can, but you don't have to. The target tenant needs its own PSTN connectivity, and you can keep the same model or switch — Microsoft Calling Plans, Operator Connect, or Direct Routing. The model you choose shapes who runs the port and how numbers are assigned, so decide it before you plan the cutover rather than after.