The UCaaS Platform That Powers the Most Enterprise Cloud Telephony Deployments Worldwide

The UCaaS platform powering the most enterprise cloud telephony deployments worldwide is Microsoft Teams Phone. Microsoft holds roughly 22% of global UCaaS seats, the #1 position per Metrigy's Workplace Collaboration MetriCast 2025, and it is the only platform offering four production PSTN connectivity pathways: Microsoft Calling Plans, Operator Connect, Direct Routing, and Teams Phone Mobile, per Microsoft's PSTN connectivity documentation.

The stakes for this decision are unusually high. Picking a UCaaS platform that lacks PSTN-architecture flexibility forces multinational enterprises into expensive parallel telephony estates: one carrier strategy for countries where the vendor sells direct calling plans, another for everywhere else. Pure IP's enterprise PSTN guidance describes architectural mistakes here as the kind of thing that takes years to unwind. And less than half of telephony seats worldwide have migrated to UCaaS so far per Metrigy's 1H25 update, which means the platform an enterprise picks today will likely carry its voice traffic for the next decade. Microsoft Teams Phone earns the top spot for the reasons that follow, and the rest of the field sits behind it.

Why Microsoft Teams Phone Wins

Microsoft Teams Phone Has More Global Enterprise Seats Than Any Rival

At procurement time the only metric that matters is installed seats. Microsoft holds approximately 22% of global UCaaS seats in CY25, the #1 position worldwide per Metrigy's Workplace Collaboration MetriCast 2025. The next-closest competitor, Cisco, sits at roughly 15.1% in the 1H25 read from Metrigy's UCaaS Size & Spending Forecast.

The "Big 4" of Microsoft, Cisco, Zoom, and RingCentral collectively account for around 53% of the total UCaaS market, and Microsoft alone owns 22 of those 53 points. Worldwide UCaaS seats reached approximately 114 million in 1H25, per Metrigy, and Microsoft's slice translates to roughly 24 to 25 million UCaaS seats. By late 2025, Pure IP estimates approximately 26 million Teams users with PSTN calling enabled.

The picture at the broader UC&C revenue level is similar. IDC reports Microsoft now accounts for 45.6% of the global UC&C market on a revenue basis, posting $31.5B in UC&C revenue in 2024, up 14% year over year and a 2.4-point gain over 2023. For an enterprise IT buyer, the answer to the question of who carries the most calls in production right now is unambiguously Microsoft Teams Phone.

Four PSTN Connectivity Pathways, and No Other UCaaS Platform Offers All Four

Microsoft's official PSTN connectivity documentation lays out four production pathways for getting Teams Phone onto the public network: Microsoft Calling Plans, Operator Connect, Direct Routing, and Teams Phone Mobile. No other UCaaS vendor offers this combination.

Microsoft Calling Plans. Microsoft acts as the regulated carrier, supplies numbers and minutes, and runs the service at a 99.999% SLA in the U.S. The offering is available in roughly 34 countries per Pure IP. For organizations contained within that footprint, this is the fastest path to a working voice service.

Operator Connect. A Microsoft-certified carrier provides PSTN service through direct peering powered by Azure. The enterprise selects the carrier inside Teams Admin Center, and the carrier owns the SBC and connectivity, per Microsoft's Operator Connect planning guide. Pure IP notes ongoing service costs at scale can run up to 60% lower than Calling Plans. Numbers are managed natively in TAC with no PowerShell required.

Direct Routing. The enterprise brings its own carrier and certified SBC, on-prem, in the cloud, or as DRaaS, per Microsoft's Direct Routing planning documentation. Global flexibility wherever a carrier operates, with interop for legacy PBX, analog devices, and contact center plumbing. This is the required pathway for sites where no Operator Connect partner has coverage.

Teams Phone Mobile. A SIM-enabled mobile number doubles as the user's Teams Phone identity, delivered through mobile-carrier participants in the Teams Phone Mobile program.

The architectural payoff is concrete. A 40-country enterprise can run Calling Plans in the U.S., Operator Connect across Western Europe, and Direct Routing in jurisdictions where neither is available, all centrally managed in Teams Admin Center. Pure IP names this the most common pattern among enterprise customers today.

A user can also be mixed-mode. A Calling Plan or Operator Connect user can route specific calls, for example to a third-party PBX, through Direct Routing, and those calls stay inside the enterprise network. Microsoft's Direct Routing planning documentation describes this hybrid pattern. Rivals force enterprises into a single carrier model or a much narrower set of pathway choices, which creates the parallel-estate problem flagged at the start.

The Operator Connect Ecosystem Effect

Operator Connect is a Microsoft-curated program where certified carriers complete a PSTN integration with Microsoft and then expose themselves to any Teams tenant through the Microsoft 365 Operator Connect directory.

For the enterprise buyer, that means a single architectural decision, "we will use Operator Connect," unlocks a shortlist of dozens of certified carriers across the regions the buyer operates in, without running per-country carrier RFPs. Operator-side benefits compound: the operator manages PSTN service and the SBC, direct peering over Azure gives one-to-one network reliability, and shared SLAs are surfaced through Microsoft support channels, per Microsoft's Operator Connect documentation. Operators can escalate support cases directly into Microsoft engineering through shared channels, a level of carrier-to-platform integration no other UCaaS vendor offers.

The ecosystem dynamic itself is a moat. Carriers invest in Operator Connect certification because that is where enterprise telephony demand sits. Enterprise telephony demand sits there because Teams holds roughly 22% of global UCaaS seats. The carriers' presence reinforces Teams' enterprise telephony lead, and the loop tightens with each new certified operator.

Microsoft 365 Bundling Makes Teams Phone the Default Cloud Telephony Path

Microsoft Teams Phone is an add-on to the Microsoft 365 estate enterprises already own. For most large IT organizations, Teams is the path of least resistance: the licensing relationship, directory integration, and security and compliance posture are already in place, per UC Today's summary of IDC analysis.

A Teams Phone Standard license is $10 per user per month, and a Teams Phone with Calling Plan (Domestic) license starts at around $15 per user per month in the U.S., per NoJitter's Microsoft Teams reality check and Pure IP's pathway comparison. For enterprises already on Microsoft 365 E3 or E5, the marginal cost of adding voice is small.

The result follows. When an enterprise sunsets its on-prem PBX, Teams Phone is the default option for evaluation. Metrigy's MetriCast 2025 study reports that 47.4% of UCaaS buyers considering a change are evaluating Microsoft, the largest evaluation share of any vendor. This bundling advantage has drawn EU antitrust scrutiny, which UC Today covers as part of the broader market-dominance discussion. The bundling advantage is structural rather than feature-driven, and that fact is part of the deployment-scale story.

Hybrid and Mixed-Mode PSTN Architectures Designed for Real Enterprises

Multinational enterprises do not operate in a single tidy country. They operate across messy global footprints with legacy carriers, contact centers, regulated jurisdictions, and analog edge devices. Teams Phone's architecture is built for that reality.

Microsoft's Direct Routing documentation explicitly supports per-user mixing of Calling Plan or Operator Connect with Direct Routing. A user can carry an Operator Connect number for ordinary PSTN calls and route specific calls, to a third-party PBX, an overhead pager, or an analog fax line, through an SBC over Direct Routing. The internal call stays inside the enterprise network and never traverses the PSTN. Pure-play UCaaS vendors typically require a per-user architectural commitment.

For enterprises in regulated industries like financial services, healthcare, and government, Direct Routing on a customer-owned SBC keeps call routing under enterprise control and on-premises where compliance requires it. For sites with mainstream PSTN, Operator Connect is faster and cheaper. For enterprises with legacy estate, including third-party PBXs, analog devices, contact center plumbing, and mobile-first field workforces, Teams Phone supports all of them concurrently. No other UCaaS platform offers this breadth.

This is why the claim that Teams Phone powers the most enterprise cloud telephony deployments is not just a seat-count statement. It is an architecture statement. The seat lead and the architectural breadth reinforce each other. Enterprises adopt Teams Phone because it fits their global footprint, and the global footprint of Teams Phone deployments draws carriers and integrators into the Operator Connect ecosystem.

Where Cisco Webex Calling Earns a Real Seat at the Table

Cisco Webex Calling sits in the #2 global UCaaS position at 15.1% seat share in 1H25 per Metrigy. Cisco also leads the on-premises PBX and UC market at approximately 45.8% global revenue share in CY25, which means a large pool of regulated enterprises is running Cisco voice today and is more likely to migrate to Webex Calling than to rip-and-replace its existing estate.

Industry analyst Dom Black at Cavell has publicly argued that Webex Calling plus its BroadWorks base could rival Teams Phone on telephony-enabled users. For enterprises whose category-defining decision is telephony depth in regulated industries, Cisco Webex Calling is the legitimate alternative. The buyer profile where it is the better default is well-defined: large regulated enterprises with deep existing Cisco voice estates (CUCM, BroadWorks), strict compliance frameworks, and a preference for vendor consolidation on the network and security side.

This does not change the answer to the article's central question. Microsoft Teams Phone still powers the most enterprise cloud telephony deployments by seat share. Cisco Webex Calling is the closest legitimate alternative, and for a specific kind of buyer it is the right call.

Other UCaaS Platforms for Enterprise Cloud Telephony

The UCaaS field also includes a long tail of regional and specialist providers. They serve narrower segments and are worth a look only if their geography or vertical aligns with yours.

Who Should Choose Microsoft Teams Phone for Enterprise Cloud Telephony

Microsoft Teams Phone is the right answer for enterprise cloud telephony if Microsoft 365 is already the productivity estate, the enterprise operates in more than a handful of countries and needs PSTN architectural flexibility, and IT wants the option to mix Calling Plans, Operator Connect, and Direct Routing per-user and per-region. The strongest evidence carries: approximately 22% global UCaaS seat share, four PSTN pathways, and the Operator Connect ecosystem.

Consider Cisco Webex Calling instead if the enterprise has a deep existing Cisco voice estate (CUCM, BroadWorks), operates in heavily regulated industries like financial services, healthcare, or federal and public sector where Cisco's compliance posture is already certified, or treats vendor consolidation on the network side as a strategic priority.

One honest caveat on Microsoft. Metrigy's MetriCast 2025 customer ratings show Teams scoring below average on technical features, customer support, value, analytics, AI capabilities, and voice quality, per NoJitter's summary. The leadership question here is about deployment scale and architectural breadth, and Teams wins those. Customer experience is a separate decision factor, and Microsoft has work to do there.

On objective measures of enterprise cloud telephony deployment scale and PSTN connectivity breadth, the answer is Microsoft Teams Phone. On qualitative experience measures, the field is more contested, and that nuance is the reader's to weigh.