Microsoft Teams Phone and Calling Plans

Microsoft Teams Phone System is Microsoft's cloud-based PBX (Private Branch Exchange) solution. It replaces traditional on-premises PBX hardware and provides call control, voicemail, auto attendants, call queues, and other telephony features directly within Teams. The Phone System is a service that runs in Microsoft 365 and is licensed per user via the Phone System add-on license (included in E5 or available as an add-on for E1/E3).

When a user makes a call, Teams client sends a SIP invite to the Microsoft 365 Phone System service. The service checks user policies (dial plan, calling policies) and determines the destination. If the call is to an internal Teams user, it routes directly via Microsoft's infrastructure. If the call is to a PSTN number, the Phone System needs a connection to the Public Switched Telephone Network (PSTN). This is where Calling Plans, Operator Connect, or Direct Routing come in. The Phone System itself does not provide PSTN connectivity; it only provides the call control. Think of Phone System as the software PBX, and PSTN connectivity as the trunk lines.

Calling Plans are Microsoft's own PSTN connectivity service. They are essentially SIP trunks provided by Microsoft's carrier partners. When you buy a Calling Plan, Microsoft assigns a phone number from its pool and provides the trunk to the PSTN. There are two types of Calling Plans:

Calling Plans are licensed per user. Each user who needs to make/receive PSTN calls must have a Phone System license AND a Calling Plan license. The Calling Plan license determines how many minutes are included. If you exceed the included minutes, you pay per minute at published rates.

Direct Routing is an alternative to Calling Plans. It allows you to connect your own PSTN carrier using Session Border Controllers (SBCs) that you manage on-premises or in Azure. The SBC connects to Microsoft Phone System via SIP trunk. This gives you control over carrier selection, number porting, and existing contracts. Direct Routing requires:

Direct Routing is more complex to set up but provides flexibility and potentially lower costs for high-volume calling.

Operator Connect is a newer option where Microsoft partners with carriers to provide PSTN connectivity directly through the Teams admin center. The carrier manages the SBC and trunks, while Microsoft handles the provisioning. This is simpler than Direct Routing because you don't need to manage SBCs. The operator provides the number and minutes, and you assign them to users via licenses. Operator Connect is available in many countries and is growing.

Teams Phone System integrates with: - Exchange Online: For voicemail transcription and storage. - Azure Active Directory: For user identity and phone number assignment. - Microsoft Teams: The client interface. - Power Automate: For call flow automation. - Compliance: Call recording can be enabled via compliance policies.

While MS-900 does not require PowerShell, you should know that administrators use:

Set-CsPhoneNumberAssignment -Identity user@contoso.com -PhoneNumber +14255551234 -PhoneNumberType DirectRouting

To verify user's phone settings:

Get-CsOnlineUser -Identity user@contoso.com | fl LineUri, EnterpriseVoiceEnabled, HostedVoiceMail

For MS-900, focus on understanding:

A multinational company with 10,000 employees is moving from a legacy Cisco PBX to Teams Phone System. They have existing contracts with a carrier for PSTN trunks. They choose Direct Routing to keep their carrier and avoid per-user Calling Plan costs. They deploy certified SBCs (AudioCodes) in two data centers for redundancy. Each SBC is configured with a public IP and TLS certificate. The voice routing policy is configured to route all international calls through the SBC in the primary data center, with failover to the secondary. They also create a tenant dial plan to normalize local 7-digit dialing to +1-XXX-XXX-XXXX. During migration, they use a hybrid approach: some users remain on the PBX while others are moved to Teams. The main challenge was ensuring the SBCs can handle the call volume (peak 500 concurrent calls) and that the network QoS is configured to prioritize Teams media traffic. Misconfiguration of the SBC's TLS settings caused call failures initially; they had to ensure the SBC's certificate chain was trusted by Microsoft's SIP proxy.

A 50-person law firm wants to use Teams for all communications. They have no existing PBX. They choose Domestic Calling Plans for all users (unlimited minutes) and one International Calling Plan for the managing partner. They purchase phone numbers from Microsoft (available numbers in their area). The admin assigns numbers via Teams admin center. They configure an auto attendant for the main line (e.g., "Press 1 for Sales, Press 2 for Support") and a call queue for support. The challenge: number porting from their old carrier took 3 weeks. Also, they needed to set up emergency addresses for each user to comply with E911 regulations. One user working remotely had an incorrect emergency address, which could delay emergency response.

A 200-person retail chain wants PSTN calling without managing SBCs. They sign up with an Operator Connect partner (e.g., AT&T). The partner provisions the trunks and provides phone numbers. In Teams admin center, the admin sees the operator's offering and assigns numbers to users. The operator handles all carrier relationships. The benefit: no SBCs to maintain, faster deployment. The drawback: less control over routing and costs. They also use Audio Conferencing for meeting dial-in. Performance is good, but they had to ensure network bandwidth is sufficient for concurrent calls during peak hours (e.g., 20 simultaneous calls). Misconfiguration of the call queue overflow caused calls to drop during lunch rush; they fixed it by increasing the queue size and setting a timeout to voicemail.