You can either use a Microsoft calling plan, access direct routing, or now take advantage of the “Operator Connect” service , Microsoft teams has enabled an exceptional UCaaS bundle of functionality and features .
Calling Plan :
An all-in-the-cloud solution with Microsoft as your PSTN carrier. offered on a per-user, per-month basis. The Calling Plans include a selection of minutes for outbound calls and a phone number for inbound calls. While Microsoft’s calling plans allow companies to leap into cloud-based telephony quickly, they have their negative points.
This solution is the simplest option that connects Phone System to the PSTN. With this option, Microsoft acts as your PSTN carrier, as shown in the following diagram:
With this option:
You get Phone System with added Domestic or International Calling Plans that enable calling to phones around the world (depending on the level of service being licensed).
You don't require deployment or maintenance of an on-premises deployment—because Calling Plan operates out of Microsoft 365.
Note: You can connect a supported Session Border Controller (SBC) through Direct Routing for interoperability with third-party PBXs, analog devices, and other telephony equipment supported by the SBC.
This option requires uninterrupted connection to Microsoft 365.
Direct Routing :
Direct Routing allows Microsoft Teams users to access their telephony solutions in more countries than you can get via Microsoft Calling Plans. Direct routing allows users to make and receive calls on Microsoft Teams by linking the platform with the PSTN (Public Switched Telephone Network) through an approved partner. You can call anyone with this service, regardless of whether they have Microsoft Teams.
The ability to choose your cloud provider comes with benefits like:
Greater phone number control: Direct routing allows companies to maintain control over their phone numbers and dial plan, rather than porting numbers directly to Microsoft. This supports faster migration to Microsoft Teams’ phone system.
Geographic reach: Microsoft is constantly expanding its calling plan availability – but it’s not available everywhere. Direct routing and Operator Connect provide international companies with more freedom to bring local calling into underserved markets.
Ability to use various cloud providers: Around 15% of companies in the study mentioned above use more than one calling platform. You can use direct routing to route your calls to various systems
Retained benefits: Sticking with your existing calling providers can be a great way to take advantage of existing deals and maintain benefits as a loyal customer. It also reduces the risk of having to switch to a new provider.
This option connects Phone System to your telephony network by using Direct Routing, as shown in the following diagram:
With this option:
You connect your own supported Session Border Controller (SBC) to Phone System without the need for additional on-premises software.
You can use virtually any telephony carrier with Phone System.
You can configure and manage this option, or it can be configured and managed by your carrier or partner (ask if your carrier or partner provides this option).
You can configure interoperability between your telephony equipment—such as a third-party PBX and analog devices—and Phone System.
This option requires the following:
Uninterrupted connection to Microsoft 365.
Deploying and maintaining a supported SBC.
A contract with a third-party carrier. (Unless deployed as an option to provide connection to third-party PBX, analog devices, or other telephony equipment for users who are on Phone System with Calling Plan.)
Operator Connect :
Operator Connect is the more recent addition to Microsoft’s list of solutions for calling functionality. This operator managed service brings PSTN calling to Teams, but in a different format to direct routing. Businesses can still get an existing PSTN carrier into the mix, provided they’re included in Microsoft’s approved list of Operators.
With Operator Connect, if your existing carrier participates in the Microsoft Operator Connect program, they can manage the service for bringing PSTN calling to Teams. Your carrier manages the PSTN calling services and Session Border Controllers (SBCs), allowing you to save on hardware purchase and management.
Operator Connect might be the right solution for your organization if:
Microsoft Calling Plan isn't available in your geographic location.
Your preferred carrier is a participant in the Microsoft Operator Connect program.
You want to find a new carrier to enable calling in Teams.
Operator Connect advantages include:
Keep existing carrier relationships: Like with direct routing, you can maintain your relationships with current carriers, provided Microsoft Teams approve them. When you’re setting up Operator Connect, you can also quickly swap between providers.
Simple deployment: It only takes minutes to connect and assign phone numbers to users. You can do everything in the Teams admin centre. You don’t need any coding expertise with PowerShell or experience with voice or DevOps. Alternatively, with Direct routing, the process is done through coding in a portal.
Streamlined management: Operator-managed infrastructure in Operator Connect means that your SBC and PSTN calling service technologies are managed by operators, reducing hardware requirements. You also get plenty of control over your service in the admin centre.
Reduced spending on hardware: Like with direct routing, you can merge communication services into teams and minimise your hardware costs. You’ll have access to both SBCs, PSTN services, and an excellent level of support from Microsoft to minimise your expenses on maintenance and service too.
The following table highlights the primary configuration differences :
References : https://docs.microsoft.com/