I am guessing the issues are intermittent? if they are actively presenting I would try to bypass the Zoom VoIP and test ST click to call to a cell phone or external emergency roll-over and see if those issues present. Hard to test during active operation hours without a reliable backup. I always hated being the middle man between telecom and ST because they often point fingers at each other or both at our internet providers. I would restart the onboarding process and look for setting changes, integration updates, hardware updates (firmware, drivers, firewalls) can be time consuming but can sometimes be easier to start over from a to z than guess where the error is.
