Plan and Design Collaboration Communications Systems
Start with the constraint, not the feature list
Design questions on MS-721 usually hinge on four cascading decisions: licensing, PSTN connectivity, network readiness, and emergency calling.
Licensing. Every user who places or receives PSTN calls needs Microsoft Teams Phone (included in E5, or the Teams Phone Standard add-on). That license is not a phone number. A number comes from one of four PSTN options: Calling Plan, Operator Connect, Direct Routing over an SBC, or Teams Phone Mobile. Choose by constraint: existing carrier contract or SBC reuse means Direct Routing; a carrier-managed service means Operator Connect; the simplest turnkey option is Calling Plan.
Network. Follow Microsoft 365 connectivity principles: local internet egress (no hairpinning through a central datacenter), no VPN or proxy for Teams media, and do not send Teams media over ExpressRoute. Size with the Network Planner, then validate with the Network Assessment Tool, Call Quality Dashboard, and Real-Time Analytics. Mark QoS: DSCP 46 audio, 34 video, 18 sharing, and make sure your SBC honors the same markings.
Emergency calling. Validated emergency locations plus uploaded network topology (subnets, Wi-Fi access points, switches, ports) drive dynamic emergency calling, so a laptop on a branch subnet resolves to the correct civic address automatically.
Rule of thumb. Solve in order: what must the user do, which license, which policy, which network path. Most wrong answers are licensing answers.
Worked example. "We have an existing SBC and carrier contract, and each site normalizes extensions differently." That is Direct Routing plus tenant dial plans and per-site voice routing policies, not a Calling Plan, which gives you no dial-plan control over the carrier.