Microsoft 365 E3 with the Phone System add-on

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Microsoft 365 Licensing: The first and foremost requirement is the appropriate Microsoft 365 licensing for the user. This typically involves a plan that includes Microsoft Teams and the necessary telephony capabilities. Common licenses that support Teams Phone include:
Microsoft 365 E5
Microsoft 365 Business Voice (for SMBs)
Microsoft 365 Business Standard with the Phone System add-on
It's vital to verify that the user has one of these qualifying licenses assigned to their account.
Phone System License: Even with a qualifying Microsoft 365 plan, a separate "Phone System" license might be required as an add-on for certain plans (like E3 and Business Standard). This license provides the core telephony features within Teams. Ensure the user has this license assigned.
Calling Plan: To make and receive calls on the Public Switched Telephone bosnia and herzegovina phone number list Network (PSTN), users need a calling plan. Microsoft offers various calling plans (Domestic, International, Pay-as-you-go) depending on your organization's needs and geographical locations. The appropriate calling plan license must be assigned to the user.

Microsoft Teams Phone Standard License: This license is often bundled within the E5 plan and is a prerequisite for utilizing features like call queues, auto attendants, and direct routing. While not always directly tied to basic number assignment, it's essential for a comprehensive Teams Phone setup.
Communication Credits: For users on pay-as-you-go calling plans or those needing to make calls to countries not included in their calling plan, Communication Credits need to be set up for the tenant and assigned or available for consumption by the user.
Common Reasons Why Phone Number Assignment Might Fail

Once the licensing prerequisites are confirmed, several other factors can prevent the successful assignment of a phone numbers to a Teams user. These can range from administrative misconfigurations to external dependencies.

Number Not Acquired or Ported: The phone number you are trying to assign must first be acquired through Microsoft or successfully ported to your Microsoft 365 tenant. If the number is not yet available within your tenant's inventory, assignment will fail.