Hi,
From your description, it may cause by the low performance.
Please check if there is any high CPU\memory\network bandwitch utilization ratio when the issue happen.
If you have deployed Monitoring service and report, then you can check if there is any call CDR report to response this issue.
Please also try to change the Response Group route method to others instead of "Parallel" to test the issue.
Make sure Lync Server update to the latest version as well.
Best Regards
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Eason Huang
TechNet Community Support
