> to my understanding, there's literally no way to eavesdrop on those calls without already compromising the server it's running on

That's probably correct (with the caveat that I suspect NSA/FSB/MSS/Mossad/whoever can reasonably be assumed to have backdoored Fortinet)

There is still the problem that an attacker with "global passive observer" capabilities (which almost certainly includes most non 3rd world nation states, and probably a few of the more problematic 3rd world ones too) can still do traffic analysis to uncover your social network (or criminal/terrorist/whistleblower/journalistic network) by identifying the call traffic endpoints.