I recently installed Cisco Teams on my laptop and the IM functionality for Lync on Outlook wasn't available. It was grayed out and the button was inactive. I spent days trying to find out the issue was. Finally I realised that the default IM application may have been changed to Cisco Teams. With the help of this article , I was able to set the default IM app back to Lync/Skype for Business. IM Providers Registry key The Registry key which stores which IM client can integrate with Outlook is: HKEY_CURRENT_USER\Software\IM Providers Below this key, you’ll find several sub keys which represent the IM clients that you have installed on your system (and registered themselves as such). In the IM Providers key, you’ll see a value called DefaultIMApp . This value should correspond to one of the sub keys. When you change the value of DefaultIMApp , restart Outlook and your new default IM application and Outlook should now use that as the application for IM...