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Consultant-focused KB for Microsoft Teams: implementation notes, gotchas and configuration decisions beyond the official docs — across chat and collaboration, meetings, webinars and town halls, Teams Phone, PSTN connectivity (Direct Routing, Operator Connect, Calling Plans, Teams Phone Mobile), auto attendants, call queues and contact centre integration, Teams Rooms and devices, apps and extensibility, Copilot in Teams, administration and governance, security and compliance, network and call quality, and licensing — plus cross-cutting decision guides.

teams/13-decision-guides/feature-decision-pstn-connectivity-model.md
feature-decision-pstn-connectivity-model.mdv1 · history
CurrentApplies to Teams PhoneUpdated 3 weeks agoSource Microsoft Learn ↗

What it does

The decision about who carries your PSTN traffic and who owns the kit in between: Microsoft (Calling Plans), a certified operator who peers straight into Teams (Operator Connect), you or a partner running a certified SBC (Direct Routing), or a mobile operator putting the SIM number into Teams (Teams Phone Mobile). Teams Phone is the PBX in every case; this is only the trunk.

Key facts

  • Every PSTN user needs a Teams Phone licence plus a PSTN solution from one of the four options. Teams Phone on its own gives you internal calling only.
  • Calling Plans: Microsoft is the operator and provides numbers, PSTN access and support, with a 99.999% SLA quoted on Learn. Requires a Calling Plan licence per user.
  • Operator Connect: needs a contract with an operator from the Operator Connect directory. The operator handles SBCs, number provisioning and porting; you enable the operator and assign their numbers in the Teams admin center. Users must be TeamsOnly.
  • Direct Routing: any PSTN carrier, connected through a Microsoft-certified SBC that you, an integrator or a DRaaS provider buys, installs and runs. The SBC needs a public IP, an FQDN on a domain registered in the tenant (not *.onmicrosoft.com) and a certificate from the Microsoft Trusted Root Program.
  • Teams Phone Mobile supports user numbers only — no Audio Conferencing numbers and no voice application (auto attendant/call queue) numbers.
  • Calling Plans, Operator Connect and Direct Routing all support user, Audio Conferencing and voice application numbers.
  • The options can be mixed in one tenant, with multiple operators, and changed over time in any order.
  • Dynamic emergency calling is supported on all four; security desk notifications on Calling Plans, Operator Connect and Direct Routing. Direct Routing needs its own emergency call routing policies.
  • Survivable Branch Appliance (branch survivability during a WAN/internet outage) is a Direct Routing feature only.
  • Since 1 November 2025, outbound PSTN calls from auto attendant and call queue resource accounts need a funded pay-as-you-go licence or Communications Credits on those resource accounts.

When to use / skip

For most UK organisations in 2026 the honest default is Operator Connect. You keep a carrier relationship (and a carrier contract you can negotiate), you don't own an SBC, numbers port through the operator, and the provisioning is a few clicks in the Teams admin center rather than a TLS certificate renewal you'll forget about in two years. There's a deep pool of UK operators in the directory, so you can usually stay with the incumbent if they're any good.

Calling Plans make sense for small or straightforward estates, distributed sites in countries Microsoft covers, and organisations that want one bill and one throat to choke. The trade-off is commercial: per-user minute bundles rather than shared carrier tariffs, a separate pool for Communications Credits, and Microsoft's porting process rather than a carrier's. If the client has heavy outbound usage, model it before you recommend it.

Direct Routing is the one you choose for a reason, not by default. Good reasons: a third-party PBX, analogue devices, door entry, lifts or fax that must stay; a contact centre integrating through the Connect model; a carrier that isn't in Operator Connect; branch survivability requirements; or a country where nothing else is available. If none of those apply, you're buying an SBC estate to run for no benefit. Teams Phone Mobile is a persona-level add-on for genuinely mobile-first staff, not a whole-estate answer — it can't carry your main number or queue numbers.

Configuration decisions

  • Default model per persona and per country: most clients end up with Operator Connect for knowledge workers, Direct Routing for the awkward bits, and maybe Calling Plans for small overseas sites.
  • Where analogue and legacy endpoints live: a Direct Routing SBC with analogue gateways, or a separate solution entirely.
  • Who owns the SBC if you go Direct Routing: client-run, partner-managed, or DRaaS. Decide who patches firmware and renews certificates, in writing.
  • Number strategy: which ranges port to which carrier, whether main numbers and queue numbers sit with a non-mobile option (they must).
  • Emergency calling design: emergency addresses, network sites for dynamic emergency calling, and who maintains them after go-live.
  • Outbound calling from voice apps: licensing the resource accounts with pay-as-you-go or Communications Credits if queues need to call out.

Gotchas

  • Teams Phone Mobile numbers can't front an auto attendant or call queue. If the client's "main number" is a mobile, that needs fixing before anything else.
  • Carrier contract terms are often the real constraint. ISDN or SIP contracts with a year left on them dictate sequencing more than any technical factor.
  • Operator Connect moves the SBC off your plate but also moves your visibility. Call-flow troubleshooting becomes a ticket to the operator.
  • Direct Routing SBC certificates expire. A lapsed certificate takes down all PSTN calling on that SBC at once, and it happens on a Friday afternoon.
  • UK non-geographic numbers (0843, 0844, 0845, 0870, 0871, 0872) can't currently be ported into Microsoft Calling Plans. Keep them with a carrier.
  • Mixing models is supported, but every extra model adds routing policies, number inventory and support paths. Two is normal; four is a support desk's nightmare.

Consultant notes

  • Start the PSTN conversation with the carrier contract end dates and the list of non-user numbers (fax, lifts, alarms, door entry). Those two lists decide the model faster than any feature comparison.
  • With the UK all-IP switchover coming, plenty of clients are being forced off ISDN anyway. Use that as the moment to consolidate onto one model rather than lift-and-shift each site.
  • If a client insists on Direct Routing "for control", ask what they'll do with it. Usually the answer is "nothing Operator Connect wouldn't give us".
  • Get the operator to confirm in writing what they handle for emergency addresses — it varies per operator.
  • Moving between models later is a number reassignment exercise, not a rebuild, so don't over-agonise. Pick the right default and leave room for exceptions.

Revisit when the Operator Connect directory changes for the UK, or if Microsoft extends Teams Phone Mobile to voice application numbers

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