Baresip is a portable and modular SIP User-Agent with audio and video support. Tries to be a Swiss Army knife for SIP and VoIP. Supports encryption. Has an embedded web server with an HTTP (REST?) API for controlling the utility. Modular architecture.
A SIP softphone for Linux.
computer desktop linux opensource oss phone software unix voice voip sip softphone
Github repo: https://github.com/DoubangoTelecom/sipml5
Jitsi is a F/OSS multiprotocol text chat/voice over IP/video chat application. It supports a large number of instant messaging and VoIP protocols, not only for IM but as transport and control channels for VoIP and video conferencing. If there's a way to get your streams through it'll do it. Natively supports OTR for protecting chats. Implements SRTP and ZRTP for encrypted conferencing. Supports DNSSEC. Aims to replace Skype as a more secure, open source package.