I found the thread replies helpful. Here are some VoIP-relevant security-related links.
The Electronic Frontier Foundation's Surveillance Self-Defence guide: https://ssd.eff.org/ In particular, this page: https://ssd.eff.org/en/module/introduction-threat-modeling Riseup has many good documents about security topics: https://riseup.net/en/security/resources The Guardian Project makes Ostel (ostel.co) and other tools to help journalists. This page is relevant to VoIP: https://dev.guardianproject.info/projects/ostn/wiki/Wiki It might be worth noting that Ostel's servers are hosted in the USA. Despite this, I like the look of this project and will probably use Jitsi+Ostel for VoIP in the future.
