How does Teams Phone work?

Teams Phone works by running your phone system as a cloud service inside Microsoft 365 rather than on a box in a wiring closet. The call-control software that a traditional private branch exchange (PBX) would provide lives in Microsoft's cloud, so calls get set up, answered, transferred, parked, and routed inside the Teams desktop, web, and mobile apps or on a certified desk phone.[1] Microsoft describes it plainly as the technology for enabling call control and PBX capabilities in the Microsoft 365 cloud, and an independent explainer frames it the same way.[1][12]

A call takes one of two paths

By default, a call between two people in the same organization is handled natively within Teams and never reaches the public switched telephone network, which removes the operator toll on internal calls.[1] A call to or from a regular phone number is different, because it has to cross from the Microsoft cloud onto the PSTN through a carrier connection. Which path a given call takes depends on whether the other party is inside your Teams organization or out on the phone network.

Two things have to line up before external calls work

A Teams Phone license and a PSTN connection are deliberately kept separate, and you need both to dial the outside world. The license entitles a user to the enhanced calling features and to access a PSTN solution, while the PSTN solution supplies the phone numbers and the actual access to domestic, international, and emergency calling.[1][3] Administrators sit on top of this with voice policies that control calling features at the org, group, and user level.[1]

The Teams Phone license is the switch that turns a chat-and-meetings user into a phone-system user. A base Teams or Microsoft 365 Enterprise license already includes native calling, meaning Teams-to-Teams calls, voicemail, and video, but the Teams Phone license is what adds the PBX features and the ability to dial PSTN numbers.[3] With it, a user gets call park, shared-line delegation, external caller ID drawn from the company directory, group call pickup, and the ability to be part of an auto attendant or call queue.[3] The license entitles the user to those enhanced capabilities and to access a PSTN solution, but it does not include any calling minutes by itself.[1][3] Some subscriptions already contain it, so a buyer on a plan that bundles Teams Phone may only need to add a PSTN connection rather than the license. On its own the license still cannot reach the outside world without the connection described next.[3][4]

The PSTN connection is what lets a Teams call cross from the Microsoft cloud onto the ordinary phone network, and Microsoft keeps it separate from the license on purpose.[1][2] A PSTN solution gives your tenant its phone numbers and its access to domestic, international, and emergency calling, and it has to be in place before any licensed user can make or receive outside calls.[2] There are four ways to supply it. A Microsoft Calling Plan makes Microsoft your carrier under a 99.999% reliability service level agreement; Operator Connect brings a certified third-party landline carrier that has already built the connection into Teams; Teams Phone Mobile ties a carrier SIM number to the Teams identity; and Direct Routing connects a carrier of your choice through a certified session border controller (SBC) that you or a partner run.[2] One tenant can mix these, for example a Calling Plan for headquarters and Direct Routing for a region where Microsoft does not sell numbers.[2] The choice changes who your carrier is and how much infrastructure you manage, not whether you need a per-user license.[2]

A licensed user still cannot place a call until two more things happen: a phone number gets assigned, and the account is voice enabled.[4] You acquire numbers through whichever PSTN option you chose, then assign a user number to a person or a service number to a resource account.[4] Service numbers matter for shared lines: a single service number can handle hundreds of calls at once, while a personal user number handles only a few, so a main line or a support line is built on a resource account.[4] Voice enabling is the second half: in the Teams admin center you turn Enterprise Voice on for the user, or you set the EnterpriseVoiceEnabled parameter with the Set-CsPhoneNumberAssignment PowerShell cmdlet.[4] Until that flag is on, the license and the number are present but the account is not cleared to use the PSTN. For large migrations, Shared Calling lets you move big groups onto Teams Phone and let them make calls before every individual number is assigned.[4]

Routing is the logic that decides where a call goes, and Teams Phone handles it in two layers: system-level voice routing for outbound PSTN calls, and voice apps for inbound calls.[6][7] On Direct Routing, an outbound call is matched against voice routing policies, which contain PSTN usages, which in turn hold voice routes; each route pairs a dialed-number pattern with one or more SBCs and sends a matching call to the right gateway, with backup SBCs tried if the first is unavailable.[6] With a Calling Plan, that routing is handled automatically by Microsoft.[6]

Auto attendants and call queues

Inbound calls to a main number usually land on a voice app rather than a person. An auto attendant answers with a menu and directs the caller to a person, a department, a call queue, an external number, or voicemail, and callers move through it by voice command or keypad, with separate handling for business hours, after hours, and holidays.[7] A call queue is the waiting room: callers hold with a greeting and music while the service looks for the next available agent.[7] Both run on resource accounts with their own service numbers rather than on a user's license.[7]

The endpoint is wherever the user actually talks: the Teams app on a computer or phone, a certified desk phone, or in many cases an existing SIP handset.[1][8] Because the calling logic is in the cloud, one phone number can ring across every signed-in endpoint at once, and a user can move a live call from a laptop to the mobile app without dropping it.[3] In practice, the Microsoft 365 integration also brings small conveniences like click-to-call straight from an Outlook contact, so dialing happens from the tools people already work in.[3][13] For organizations that already own phones, SIP Gateway lets compatible SIP devices from vendors such as Poly, Yealink, Cisco, and AudioCodes sign in to Teams and make and receive calls, so existing hardware keeps working.[8] A SIP device used this way acts as a Teams endpoint and can reach the PSTN, other SIP devices, and Teams users, though the user still needs a phone number with PSTN calling enabled.[8] The same cloud service backs all of these endpoints, so you can fit the device to the role, from a headset for a remote agent to a common-area phone in a lobby.[3][8]

Once the cloud service has set up a call, the two things moving between endpoints are signaling and media, and they take different paths.[5] Signaling is the control channel that starts the call and carries instructions; it mostly uses HTTPS-based interfaces and always goes to the closest Microsoft 365 server to the user, and it uses SIP where the connection reaches a session border controller.[5] Media is the actual audio, carried in Real-time Transport Protocol packets and secured with SRTP so the payload is encrypted.[5]

Media takes the most direct path it can

For a call between two people, the media tries to take the most direct route available.[5] Teams uses standard Interactive Connectivity Establishment (ICE) procedures: each endpoint gathers candidate addresses, the two sides run connectivity checks, and the best working path wins, favoring UDP because it is the better transport for real-time audio.[5] When a direct path is blocked, for example between an office user and someone on the open internet, a Transport Relay inside Microsoft 365 relays the media instead.[5] With Direct Routing, media bypass can keep the audio flowing straight between the client and the on-premises SBC rather than routing it up through the cloud, which shortens the path for a PSTN call.[3][5]

Emergency calling works by attaching a validated physical address to a user or a location so that responders receive a usable dispatchable address when someone dials for help.[9] Microsoft recommends creating addresses with the map search feature in the Teams admin center, which validates the address and attaches the geo codes that some countries use to route the call to the correct public safety answering point.[9] A registered address is the static location assigned to a user, which fits people at a fixed desk.[9] Dynamic emergency calling goes further by detecting the current network location of the Teams client and routing both the call and any security-desk notification based on where the person actually is, and it is supported across all four PSTN options, though Direct Routing needs extra configuration.[9] How much of this you set up yourself depends on the connection: with a Microsoft Calling Plan the emergency screening service and registered address are included, while other options hand more of the responsibility to the carrier or your own team.[2][9] For staff working from home, enabling location sharing lets a remote emergency call carry a current address.[9] United States deployments also carry regulatory emergency-calling obligations that vary by PSTN option.[2][9]

Teams Phone depends on your internet, your licensing choices, and correct configuration, and each of those shapes a limit worth planning around.

The license is not the whole bill

A Teams Phone Standard license lists at $10.00 per user per month paid yearly, but on its own it cannot call the outside world, so a working deployment also needs a PSTN option: a bundle such as Teams Phone with Calling Plan runs about $17 per user per month, and independent pricing guides current to April 2026 match those figures and note you still need a base Teams or Microsoft 365 license underneath.[10][11] Budgeting for the phone license alone understates the real cost.

It runs on data, power, and the network

Because calls travel as encrypted media over IP, a call needs a working internet connection and power at the user's location, unlike a legacy analog line that drew power from the phone company.[5] Quality tracks the network, so a congested link or a VPN sitting in the media path can degrade a call.[5]

Some routing has to be built and maintained

Direct Routing gives you the most control, but the number patterns, voice routes, and SBC itself have to be configured and kept current, which asks more of your team than a Microsoft-managed Calling Plan does.[2][6]

Teams Phone is the calling layer of Microsoft 365, not a standalone app or a phone carrier. A few common mix-ups are worth clearing up.

It is not the same as basic Teams calling

Every Teams user can already call another Teams user over the internet without Teams Phone.[3] The Teams Phone license is the separate piece that adds PBX features and, with a PSTN connection, the ability to dial real phone numbers.[1][3]

It is not a phone carrier on its own

Teams Phone provides the phone system, not the numbers or the line to the outside network. Those come from a PSTN option, and with three of the four options a third party rather than Microsoft is your actual carrier.[2]

It is not tied to one type of phone

The service backs Teams apps, certified desk phones, and compatible existing SIP devices through SIP Gateway, so moving to it does not require throwing out every handset you own.[8]

It is not a guarantee of your call quality

Microsoft's 99.999% service level agreement covers the Calling Plan service, not your local internet, so a working deployment still depends on the network between the endpoints.[2][10]