How Teams connects to the telephone network

When implementing Microsoft Teams as a telephone system, the choice in many projects is made based on the setup effort or along what the previous provider has to offer. However, this can be tricky, because both are weak criteria for the implementation itself as well as for the adoption of the new system in the company. The reverse order is more reliable: first the existing setup, then the connection; because only if the new system does not put an additional burden on employees (but ideally even relieves them) will they accept it and get used to it.

This article explains the three options and names the points on which the decision actually depends.

What distinguishes the three connection methods

Microsoft Calling Plans: Microsoft itself is your telephone provider. Phone numbers come from the Microsoft inventory or are ported to Microsoft from your existing contract partner. No proprietary telephone system infrastructure is required anymore, nor is a third contract partner. Availability varies by country, as do the available phone number types. In principle, Microsoft Calling Plans are the "simplest" solution, but also the least flexible, as you are dependent on Microsoft's native integrations and compatibility tools for all connections.
The management of the system is comparatively simple and takes place entirely in the Microsoft Teams Admin Center - so it can be administered by your existing M365 team.

With Direct Routing, you operate a certified SBC (Session Border Controller) yourself in your data center or the cloud, or obtain it as a managed service to network a wide variety of systems, map complex rule sets, and distribute calls among each other. Practically any SIP provider can be connected to it, and you can usually continue to use your existing connection without complex phone number porting and contract negotiations, with your own control over routing and system boundaries.
It is an extremely flexible integration path that also allows the parallel operation of different telephone systems (including Teams Phone) on the same connection - with corresponding operating and implementation effort (planning, configuration, network optimization, etc.), the more complex the infrastructure to be connected is. The day-to-day configuration tasks also take place in the Teams Admin Center here, but for larger changes or those affecting multiple systems, adjustments to the SBC are usually also necessary.

With Operator Connect, on the other hand, the telephone provider operates the PSTN connection including the Session Border Controller and provides your phone numbers directly in your Microsoft tenant. Peering to Microsoft takes place directly at the provider and is subject to high requirements and certification programs for the providers. You obtain phone numbers from the operator and assign them in the Teams Admin Center. For your IT team, this is virtually just as easy as Calling Plans, but Operator Connect providers offer other, far more flexible pricing models and sometimes also the system-side integration of existing systems or endpoints.

For everyday telephony, all three lead to the same result. Differences are mainly evident in price, implementation effort, and the ability to integrate third-party systems. That is where it is decided which variant best fits your environment.

Consideration 1: What is still connected to the old system?

As a rule, more devices than just desk phones are connected to an established telephone system. Door intercoms, elevator emergency calls, a fax machine in accounting, DECT handsets in the warehouse and production, occasionally an extension in the neighboring building. These devices do not speak "Teams". They need an integration that can be provided by the SBC or a gateway - or an alternative path into the telephone network.

Direct Routing is the most flexible way for this because the SBC is in your own access or your managed service partner takes care of it, but it often also requires the management of multiple stakeholders and service providers.
Various Operator Connect operators now also offer hybrid scenarios with analog devices, splitting of phone number blocks, or contact center connections, depending on the operator. It is important to consider this question right at the beginning of your strategic planning in order to avoid short-term project stops due to "crap, we completely forgot about that, it won't work like this" shortly before go-live.

Consideration 2: Operators, contracts, and deadlines

Whether your current operator is listed in the Operator Connect program can be clarified in the Teams Admin Center in a few minutes. If they are, you can continue to use existing contracts and phone numbers without porting to Microsoft. If they are not, a change of operator or Direct Routing (self-hosted or as a managed service) are options. This is primarily a commercial question that can be answered with a look at the current contracts with your provider.

Consideration 3: Who operates?

Calling Plans: Everything lies with Microsoft. You only need to ensure the internet connection of the clients. Everything is configured in the Teams Admin Center.

Operator Connect: The telephone provider takes care of everything. You manage telephony in the Teams Admin Center without additionally required configurations such as Voice Routing Policies or Voice Routes.
"Like Calling Plans - just more flexible and usually cheaper".

Direct Routing: An SBC stands at the network boundary. It needs patches, certificate changes, and monitoring, and in the event of an error, additionally someone who can read a SIP trace. If this role is not filled in the team and is not intended to be filled, self-operation is not necessarily wrong, but risky. Direct Routing as-a-Service (available directly from providers or managed service partners) covers such cases without you having to sacrifice flexibility.

Consideration 4: Network connection?

Calling Plans: Everything lies with Microsoft. You only need to ensure the internet connection of the clients. Everything is configured in the Teams Admin Center.

Operator Connect: Here too: only the connection to Microsoft 365 is relevant for customers. The connection to the telephone network is established directly between Microsoft and the telephone provider.

Direct Routing: An SBC stands at the network boundary. Depending on where it is hosted, special attention must be paid to the network connection in the direction of the affected location to Microsoft AND the telephone provider: SD-WAN, Multi-WAN, failover routing, high availability of SBCs. Because if your SBC is local and it or the internet connection fails, the entire telephone system is no longer available.

Consideration 5: Combinations of the three variants?

The connection is determined per location or phone number (block), not per tenant. Multiple variants in parallel are not the exception, but occur regularly.

Example: During a migration at the main location, an SBC networks the existing telephone system and Microsoft Teams in order to be able to move users to Teams department by department. Branch locations are provided using Operator Connect or Calling Plans, as these locations do not require the flexibility of Direct Routing.

Calls to the old system remain in the proprietary network. In the long run, therefore, one often splits up, unless one wants to or can limit oneself to one path or telephone provider for all locations. Operator Connect where the operator covers the required features, Direct Routing for locations with analog technology, non-replaceable infrastructure. Calling Plans can, for example, also make sense for smaller branch locations or remote business units, as the management effort is very low.

tl;dr: Which connection method fits which environment

If analog endpoints, a third-party PBX, or a SIP-bound contact center are involved, the path usually leads to Direct Routing, in case of doubt as a managed service.

If the existing setup is manageable, the operator is listed, and/or SBC operation is not desired, Operator Connect is the shorter path.

Calling Plans are suitable for small branch locations and environments without special requirements in supported countries.

With multiple locations, it can therefore easily result in a combination.

The connection question can now be roughly answered; the effort in migration projects lies in the inventory beforehand:

Which phone number blocks exist?
Which systems and devices are connected where?
Which contracts with which deadlines are running?
Who manages what?

When you implement Microsoft Teams as a phone system, you’re not only choosing a new technology with many new features and improvements, but also determining how smoothly your employees’ day-to-day work will run in the future: If you truly understand your current setup first, you’ll ultimately find the integration option that works best for you - both functionally and financially.

Cloudpandas accompanies medium-sized companies in converting their telephony to Microsoft Teams and in everything that comes after: from the phone number to the voice assistant that really thinks along.