To assign a phone number and enable voice services for a user in a Microsoft Teams or Skype for Business environment via PowerShell, the Set-CsUser cmdlet is the primary tool.

1. Box 1: Set-CsUser is the correct cmdlet because it modifies the attributes of an existing user account, specifically those related to communication services.
2. Box 2: The -LineURI parameter is used to specify the user's phone number. The value must be in the E.164 format and prefixed with "tel:" (e.g., tel:+61370105550), as shown in the command snippet.
3. EnterpriseVoiceEnabled: This parameter must be set to $true in conjunction with a LineURI for the user to successfully send and receive calls using the Phone System.

Microsoft Learn Documentation: Set-CsUser cmdlet reference. URL: https://learn.microsoft.com/en-us/powershell/module/skype/set-csuser

Microsoft Learn Documentation: Assign, change, or remove a phone number for a user. Section: "Using PowerShell to assign a phone number".

Microsoft 365 Certified: Teams Administrator Associate (MS-700): Exam objective "Manage Phone Numbers" specifically covers the use of the Set-CsUser cmdlet for LineURI assignment.