Teams and your existing PBX: coexistence and gradual migration

How coexistence works

Direct Routing is designed to let Teams connect to existing PBXs, analogue devices and SIP trunks via a certified SBC.

• Mitel supports Direct Routing from MiVoice Business, MiVoice 5000, MX-ONE and OpenScape, so you keep your operator contracts, numbers and SIP trunks.
• OpenScape 4000: the manufacturer’s documentation describes interworking with Teams via AudioCodes SBC over the PBX’s built-in SIP trunks.
• OpenScape Voice: AudioCodes has a configuration guide for connecting the PBX to Teams via Mediant SBC.
• Without Direct Routing, there is Mitel Assistant, a tab in Teams with two-way presence, call history and directory. Telephony then stays in the Mitel PBX.

A shared numbering plan

With Teams Phone via Direct Routing, you keep your contract with the operator for your numbers. This is how it is usually done during coexistence: the number range comes in to the SBC, which decides for each number whether the call goes to Teams or to the PBX. When a group moves, the routing for their numbers changes and they get their number in Teams. Internal calls between Teams and the PBX go via the SBC, which translates between extensions and full numbers.

Step by step

• Assessment: PBX, number ranges, operator contracts, contact centre, analogue devices and alarms.
• SBC and connection to Teams: domain, certificate, firewall and routing.
• Pilot with a test group: inbound and outbound calls, emergency calling, failover routes and call quality.
• Migration group by group, one department or office at a time.
• Decommissioning of the PBX once the last functions have moved or been replaced.

Users who will make calls via Direct Routing must have their account in the cloud and use Teams as their only client for calls and chat.

What often stays on the PBX

• Contact centres built on the PBX can stay and connect to Teams via the SBC. Agents working in Mitel CX can collaborate internally in Teams.
• Analogue devices such as alarms, lifts, door entry phones, fax machines and emergency phones can stay on the PBX or connect via an SBC or an analogue adapter.
• DECT and other on-site wireless phones can in many cases connect to Teams, but need checking model by model.
• Attendant consoles with advanced presence and diversion status, and features that Teams lacks.

Give each such function an owner and a decision: move, replace or keep.

Pitfalls

• Emergency calling (112): with Direct Routing, the route and policy for the emergency number must be set up and tested, and the address must be registered with the operator for every number.
• Survivable mode in branch offices: emergency calls made in survivable mode do not send location information.
• The web client does not support all emergency calling and media path features. Consider disabling calls to the phone network there.
• Certificates: the SBC must trust Microsoft’s new root certificates, and your own certificates must be renewed more and more often.
• Software outside a certified version can lead Microsoft to reject support cases.
• Transcoding between audio formats uses SBC capacity.

Last updated 14 October 2026.