Troubleshooting dropped PSTN calls - how?

New Contributor

We are using Teams Phone System with MS Calling plans, across a variety of locations, networks, with certified native Teams desk phones (both Yealink and Poly CCX). No Direct Routing, no SBCs. PSTN Calls drop frequently, and under a variety of conditions, and to date nobody has been able to tell us why. It can occur with direct dialed calls in either direction to the PSTN, or on calls picked up via Call Queues. The situation has improved marginally over the years, but it's still unreliable, years after adoption.

 

The usual approach for diagnosing issues that is suggested is to look at the CQD - but of course, it only tells us that the quality was good, and that the call completed normally (which it did not) so clearly there is some level of logging involved that's not taking into account what happens in these circumstances. Furthermore, we've often been instructed to pull logs from the various phones themselves, but every single time we go through that exercise with MS technical support, they can find absolutely nothing wrong. For what it's worth, running network assessments at the locations involved all pass with flying colors, there are no ports blocked by firewalls or otherwise, latency, jitter, buffer-bloat, packet loss, all extremely low. QOS is being used.  The network to the best of my knowledge is never implicated. 

Someone must have better insight into this than I. Clearly, if a call drops, and the CQD or call diagnostics reports from MS say "Everything is great" then either there is a problem that occurs externally that they are not detecting, or, they simply aren't collecting enough data to know what the problem is.

When a call drops, and the CQD indicates nothing obvious, is there somewhere else that we can investigate for more information? What do other people do in this situation? 

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@CIVIC_Infusion 

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