Hi @Mark TT ,
Does this issue exist all the time?
Does anyone else have the same issue in your organization?
Do you have any operations before this issue occurred?
If this issue only exists in specific user, we recommend you try to disable this user in Skype for Business Server and re-enable it to see if it can be fixed.
In addition, please try to change another device to see if it can be reproduced.
At the same time , please check if there is any error messages from the event log in Skype for Business Server.
If the response is helpful, please click "Accept Answer" and upvote it.
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