Short answer
The FirstNet SLA for Microsoft Teams calling covers the parts FirstNet manages: PSTN links, the SBC platform, number provisioning and FirstNet's support response. Microsoft's own services stay with Microsoft, but FirstNet runs the joint escalation with Microsoft for you.
In detail
What sits where:
- Covered by FirstNet: PSTN connectivity and call termination, plus the SBCs (session border controllers) it manages. The FirstNet SIP trunk, number provisioning and support are covered too.
- Up to Microsoft: the Teams service itself, the Phone System feature and the uptime of your Microsoft 365 tenant.
- Outside the SLA: any third-party carrier or internet link you buy yourself.
- Joint escalation: FirstNet's Operator Connect or Direct Routing support channel is your one point of escalation. That includes Microsoft issues.
FirstNet's service-specific terms and your order form set out service credits, uptime targets and measurement windows. With Direct Routing, an SBC pair with failover or clusters in different locations add resilience. They do not change the contract SLA. The service schedule governs that.
Call quality depends on the internet path between your site and Microsoft 365. So FirstNet recommends pairing Teams calling with managed connectivity, ideally with a backup link. Operator Connect also uses direct peering with Microsoft through Azure Peering Services, not best-effort internet.
Source: FirstNet Microsoft Teams Calling service page →
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