You need to troubleshoot the issue

A company has an Office 365 tenant and uses Exchange Online and Skype for Business Online.
User1 is scheduling a Skype meeting with User2.
User 1 is not able to see availability information for User2.
You need to troubleshoot the issue.
What should you use?

A.
Microsoft Lync Connectivity Analyzer Tool

B.
OCSLogger

C.
ClsController

D.
Remote Connectivity Analyzer

Explanation:
Problem itself is related to DNS

https://support.microsoft.com/en-us/kb/2565604