Not directly related, but we had an issue this week where when if one was using the Zoom Meeting's "Call Me" method of joining the call audio, if the user used their Zoom Phone number, the call would not come through and would not get logged by Zoom Phone either. In Zoom Meeting it would say the number was busy (though was not).
Turned out to be some unspecified mistake in their "security configuration" that got changed over the weekend so the calls never made it to Zoom Phone.
If nothing else, maybe you can test using the "Call Me" feature and - if can make the issue occur that way, you can then have them check their Zoom Meeting PSTN call logs on their end....