What’s kind of your environment?
According to your description, it seems that your environment is hybrid. If so, please make sure that you assign Microsoft Teams license and Skype for Business Online (Plan 2) license to affected users.
If it has no luck, I want more detailed information on this problem:
When did this problem happen? How often does it happen to users?
How many users experience this?
What changes do you do before this happen?
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