You administer an Office 365 deployment.
You need to ensure that users can dial telephone numbers by using either the Skype for Business app for
Windows or the Skype for Business mobile app.
What should you do? telephony infrastructure.

A.
In the Office 365 Admin center, enable Skype for Business PSTN Calling.

B.
Run the Windows PowerShell cmdlet Enable-CsOnlineDialInConferencingUser.

C.
In the Office 365 Admin center, enable Skype Meeting Broadcast.

D.
Run the Windows PowerShell cmdlet Set-CsUser.

E.
In the Office 365 Admin center, enable SkypeCloud PBX.

Explanation:

To enable calls to landlines and mobile phones, Phone System can be connected to the PSTN by purchasing
Calling Plan, or using on-premises PSTN connectivity, where software on-premises connects with your existing
References: https://technet.microsoft.com/en-us/library/mt612869.aspx