You are the Office 365 administrator for your company. Users report that they cannot sign in to Skype for Business from

[answerhappygod]

You are the Office 365 administrator for your company.
Users report that they cannot sign in to Skype for Business from their mobile devices, but they are able to send and receive Skype for Business messages by using their laptop computers.
You need to troubleshoot the issue.
What should you do?

A. From the Office 365 message center, confirm Skype for Business settings.
B. Use the Microsoft Connectivity Analyzer tool to confirm settings.
C. Confirm Skype for Business user licenses for the affected users.
D. From the Skype for Business admin center, verify the external access settings.