Hi Tony!
Does this issue persist all the time?
Does anyone else have the same issue?
You can try to check if EWS status and MAPI status are OK. Please press Ctrl and right click Lync icon in the taskbar, open configuration information. Then you can see the following picture:

You need to ensure that both Exchange 2013 and Skype for Business Server 2015 are fully installed, and then assign a server-to-server authentication certificate for both exchange and SFB, after this, configure autodiscover
on Exchange. For more details about deploy EWS, you can refer to:
https://docs.microsoft.com/en-us/skypeforbusiness/plan-your-deployment/integrate-with-exchange/integrate-with-exchange?redirectedfrom=MSDN
Best Regards,
Jimmy Yang
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