@danfeng 

To add to @Christopher Hoard response of QoS, adding QoS to the signalling portion of the call wouldn't change the call quality or performance of the call as that's just part of the initial setup.  I'd concentrate on just QoS on the media portions.

 

If you do still want to put QoS into place on the signalling portion, you can explicitly define the source/targets as well, listed on the Office365 URLs and IPs page https://docs.microsoft.com/en-us/microsoft-365/enterprise/urls-and-ip-address-ranges?view=o365-world...

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