Explanation

Reference:
https://docs.microsoft.com/en-us/microsoftteams/coexistence-chat-calls-presence

Explanation/Reference:
Plan and Configure a Microsoft Teams Environment
Testlet 2
Case study
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Overview
Contoso, Ltd. is a pharmaceutical company that has 3,000 users in offices across North America.
Existing Environment
Active Directory
The network contains an on-premises Active Directory domain. All user accounts are in departmental organizational units (OUs).
Microsoft Office 365
All users are assigned a Microsoft Office 365 E5 license and are enabled for the following services:
* Microsoft Yammer
* Microsoft Exchange Online
* Microsoft SharePoint Online
* Microsoft OneDrive for Business
* Azure Active Directory (Azure AD)
* Azure AD dynamic group membership
Microsoft Skype for Business
Contoso has an on-premises deployment of Microsoft Skype for Business Server 2015. All the latest cumulative updates are installed on all the servers. The following services are used in Skype for Business Server 2015:
* Meetings
* PSTN calling
* Instant messaging (IM)
* Federation to external companies
Departments
Contoso has the following departments:
* IT
* Sales
* Human resources (HR)
* Research and development (R&D)
All groups are created by using dynamic group membership that use the department attribute.
Requirements
Business Goals
Contoso identifies the following business goals:
* Migrate the maximum number of users to Microsoft Teams.
* Minimize administrative effort.
Device Requirements
Contoso identifies the following device requirements:
* Replace Skype for Business IP phones with Microsoft Teams phones.
* All Microsoft Teams phones must be set to lock automatically after 60 seconds.
Meeting and Live Event Requirements
Contoso identifies the following meeting and live event requirements:
* Requirement1: Guest users must be able to join live events.
* Requirement2: Guest users must always wait in a meeting lobby.
* Requirement3: Contoso users must always bypass the meeting lobby when they join meetings by using the Microsoft Teams client.
* Requirement4: After the upgrade to Microsoft Teams, all meetings must be scheduled by using Microsoft Teams.
Calling Requirements
Contoso identifies the following calling requirements:
* Migrate all existing auto attendants and response groups to Office 365.
* Minimize the on-premises PSTN infrastructure.
Technical Requirements
Guest users must be prevented from making private calls.
Security Requirements
Contoso has the following security requirements for the Microsoft Teams deployment:
* The number of ports allowed on the company's firewall must be limited.
* Team names must be prevented from containing the name of any products produced by Contoso.
* Users in the R&D group must be prevented from sending chat messages to users in other departments.
* To meet the company's compliance policy, users in the sales department must continue to use on-premises Skype for Business for IM and calling only.
Pilot Project
Contoso begins a pilot project for the upgrade to Microsoft Teams.
You identify the following issues during the pilot project:
* Microsoft Teams users report that they often receive continuous notifications from the Microsoft Teams client for 20 minutes.
* Two pilot users in the HR department report that they are not members of the HR team.