This could be resolved by the communications replacing certificates:
"All,
Friday morning [1/11/2019] we replaced certificates on :
Call Manager Publisher \ Subscriber
Unity Voice Mail Publisher \ Subscriber
IM & Presence (Jabber) Publisher \ Subscriber
Emergency Responder Publisher \ Subscriber
Express Way Series Core \ Edge – Publisher \ Subscriber
There were a few things that happened that were not fully expected.
1.Reboot of a service took everyone off Jabber for about 45 seconds of interrupted service
2.Phones \ Jabber users whom logged out or reset were unable to get into Jabber \ Phones"
After that's been done, do the following:
-Clear office credentials in windows credential manager
-Clear the outlook cache by disabling/renabling cache mode via outlook
-Applied an Outlook 2016 registry fix and opened the registry fix on the user's profile (\\ladco\shares\software\Microsoft\Outlook\Autodiscover Registry Fix)
After doing the above, that should resolve the issue.