If there is a missing Skype/Lync button missing for user in Outlook, following has to be done under user’s profile:

  1. Click Start, type in “Outlook”, right click on it and Run as administrator
  2. In Outlook, click File, Options, Add-Ins.
  3. On the Manage menu, select COM Add-ins and click Go
  4. Here find the add-in (can be named Skype Meeting Add-in or Lync Meeting Add-in), select it and click Remove
  5. After that, click Add and navigate to:
C:\Program Files (x86)\Microsoft Office\Office16 and select UCAddin.dll

(if you can’t find the UCAddin.dll there, search in File explorer for the UCAddin.dll in C:\Program Files (x86)\Microsoft Office\ folder)

Click OK, check if the Add-in is now enabled, click OK close Outlook, restart the machine and let user check if the issue is now resolved. There is no need to run the Outlook as admin anymore.