Seth

I haven't found a reliable, usable, simple, free software VoIP
client for Linux

This is the best I see. For most of the platforms.
https://jitsi.org/Main/Download
https://github.com/jitsi/jitsi
 
From their page:

Encrypted password storage
Password protection with a master password
Encrypted Instant Messaging with Off-the-Record Messaging (OTRv4)
Chat authentication with the Socialist Millionaire Protocol over OTR
Call encryption with SRTP and ZRTP for XMPP and SIP
Call encryption with SRTP and SDES for XMPP and SIP
DNSSEC support

TLS support and certificate-based client authentication for SIP and XMPP